Redox flow secondary battery and electrolyte membrane for redox flow secondary battery

ABSTRACT

The purpose of the present invention is to provide a redox flow secondary battery which has low electrical resistance and excellent current efficiency in addition to durability. The present invention relates to: an electrolyte membrane for redox flow secondary batteries, which contains an ion exchange resin composition containing a fluorine-based polymer electrolyte; and a redox flow secondary battery which uses the electrolyte membrane for redox flow secondary batteries.

TECHNICAL FIELD

The present invention relates to a redox flow secondary battery, and anelectrolyte membrane for a redox flow secondary battery.

BACKGROUND ART

Redox flow secondary batteries are to store and discharge electricity,and belong to large-size stationary batteries used for leveling theamounts of electricity used. The redox flow secondary battery isconfigured such that a positive electrode and an electrolyte solutioncontaining a positive electrode active substance (positive electrodecell) and a negative electrode and a negative electrode electrolytesolution containing a negative electrode active substance (negativeelectrode cell) are separated by a separation membrane; charge anddischarge are carried out by utilizing the oxidation and reductionreactions of both the active substances; and the electrolyte solutionsincluding both the active substances are circulated from storage tanksto an electrolytic bath, and a current is taken out and utilized.

As an active substance contained in an electrolyte solution, there areused, for example, iron-chromium-based ones, chromium-bromine-basedones, zinc-bromine-based ones, and vanadium-based ones utilizing thedifference in electric charge.

Particularly, vanadium-type secondary batteries, since having advantagesof a high electromotive force, a high electrode reaction rate ofvanadium ions, only a small amount of hydrogen generated as aside-reaction, a high output, and the like, are being developedearnestly.

For separation membranes, devices are made so that electrolyte solutionscontaining active substances of both electrodes are not mixed. However,conventional separation membranes are liable to be oxidized and forexample a problem thereof is that the electric resistance needs to bemade sufficiently low. Although in order to raise the current efficiencyof batteries, the permeation of each active substance ion contained inthe cell electrolyte solutions of both the electrodes (contaminationwith electrolytes in electrolyte solutions of both electrodes) isdemanded to be prevented as much as possible, an ion-exchange membraneexcellent in the ion permselectivity, in which protons (H⁺) carrying thecharge easily sufficiently permeate, is demanded.

The vanadium-type secondary battery utilizes an oxidation and reductionreaction of divalent vanadium (V²⁺)/trivalent vanadium (V³⁺) in anegative electrode cell, and oxidation and reduction reaction oftetravalent vanadium (V⁴⁺)/pentavalent vanadium (V⁵⁺) in a positiveelectrode cell. Therefore, since electrolyte solutions of the positiveelectrode cell and the negative electrode cell contain ion species ofthe same metal, even if the electrolyte solutions are permeated througha separation membrane and mixed, the ion species are normally reproducedby charging; therefore, there hardly arises a large problem as comparedwith other metal species. However, since active substances becominguseless increase and the current efficiency decreases, it is preferablethat the active substance ions freely permeate as little as possible.

There are conventionally batteries utilizing various types of separationmembranes (hereinafter, also referred to as “electrolyte membrane” orsimply “membrane”); and for example, batteries are reported which useporous membranes allowing free permeation by an ionic differentialpressure and an osmotic pressure of electrolyte solutions as the drivingforce. For example, Patent Literature 1 discloses apolytetrafluoroethylene (hereinafter, also referred to as “PTFE”) porousmembrane, a polyolefin (hereinafter, also referred to as “PO”)-basedporous membrane, a PO-based nonwoven fabric, and the like as aseparation membrane for a redox battery.

Patent Literature 2 discloses a composite membrane in combination of aporous membrane and a hydrous polymer for the purpose of the improvementof the charge and discharge energy efficiency of a redox flow secondarybattery and the improvement of the mechanical strength of a separationmembrane thereof.

Patent Literature 3 discloses the utilization of a membrane of acellulose or an ethylene-vinyl alcohol copolymer as a nonporoushydrophilic polymer membrane excellent in the ion permeability andhaving a hydrophilic hydroxyl group for the purpose of the improvementof the charge and discharge energy efficiency of a redox flow secondarybattery.

Patent Literature 4 states that the utilization of a polysulfone-basedmembrane (anion-exchange membrane) as a hydrocarbon-based ion-exchangeresin makes the current efficiency of a vanadium redox secondary battery80% to 88.5% and the radical oxidation resistance excellent.

Patent Literature 5 discloses a method of raising the reactionefficiency by making expensive platinum to be carried on a porous carbonof a positive electrode in order to raise the current efficiency of aredox flow secondary battery, and describes a Nafion (registeredtrademark) N117 made by Du Pont K. K. and a polysulfone-basedion-exchange membrane as a separation membrane in Examples.

Patent Literature 6 discloses an iron-chromium-type redox flow batteryin which a hydrophilic resin is coated on pores of a porous membrane ofa polypropylene (hereinafter, also referred to as “PP”) or the like. AnExample of the Patent Literature uses a membrane covered in a thicknessof several micrometers with a fluorine-based ion-exchange resin (made byDu Pont K. K., registered trademark: Nafion) on both surfaces of a PPporous membrane of 100 μm in thickness. Here, Nafion is a copolymercontaining a repeating unit represented by —(CF₂—CF₂)— and a repeatingunit represented by —(CF₂—CF(—O—(CF₂CFXO)_(n)—(CF₂)_(m)—SO₃H))— whereinX═CF₃, n=1, and m=2.

Patent Literature 7 discloses an example of a vanadium-type redox flowsecondary battery decreased in the cell electric resistance as much aspossible and raised in the efficiency by the improvement of theelectrode sides including the usage of a two-layer liquid-permeableporous carbon electrode having a specific lattice plane.

Patent Literature 8 discloses an example of a vanadium-type redox flowbattery using an anion-exchange type separation membrane having a lowmembrane resistance, being excellent in the proton permeability and thelike, and being composed of a crosslinked polymer having a pyridiniumgroup (utilizing N⁺ as a cation). The crosslinked polymer disclosed is apolymer obtained by copolymerizing a pyridinium group-containing vinylpolymerizable monomer, a styrene-based monomer and the like, and acrosslinking agent such as divinylbenzene.

Patent Literature 9 discloses a redox flow secondary battery using aseparation membrane which is made by alternately laminating acation-exchange membrane (fluorine-based polymer or anotherhydrocarbon-based polymer) and an anion-exchange membrane(polysulfone-based polymer or the like), and which has a cation-exchangemembrane disposed on the side of the separation membrane contacting witha positive electrode electrolyte solution, for the purpose of reducingthe cell resistance and improving the power efficiency and the like.

Patent Literature 10 discloses a secondary battery using as a separationmembrane a membrane excellent in the chemical resistance, low in theresistance, and excellent in the ion permselectivity, which is ananion-exchange membrane made by compositing a porous base materialcomposed of a porous PTFE-based resin with a crosslinked polymer havinga repeating unit of a vinyl heterocyclic compound having two or morehydrophilic groups (vinylpyrrolidone having an amino group, or thelike). The principle described therein is that although metal cations,having a large ion diameter and a much amount of electric charge,receive an electric repulsion by cations of a separation membranesurface layer part and are inhibited from the membrane permeation underthe potential difference application, protons (H⁺), having a small iondiameter and being monovalent can easily diffuse and permeate in theseparation membrane having cations to thereby give a low electricresistance.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Laid-Open No. 2005-158383-   Patent Literature 2: Japanese Patent Publication No. H6-105615-   Patent Literature 3: Japanese Patent Laid-Open No. 562-226580-   Patent Literature 4: Japanese Patent Laid-Open No. H6-188005-   Patent Literature 5: Japanese Patent Laid-Open No. H5-242905-   Patent Literature 6: Japanese Patent Laid-Open No. H6-260183-   Patent Literature 7: Japanese Patent Laid-Open No. H9-92321-   Patent Literature 8: Japanese Patent Laid-Open No. H10-208767-   Patent Literature 9: Japanese Patent Laid-Open No. H11-260390-   Patent Literature 10: Japanese Patent Laid-Open No. 2000-235849

SUMMARY OF INVENTION Technical Problem

However, the battery disclosed in Patent Literature 1 is not sufficientin the electric resistance and the ion permselectivity of the separationmembrane, and is insufficient in the current efficiency, the durability,and the like.

The composite membrane disclosed in Patent Literature 2 has a highelectric resistance, and a problem thereof is that each ion freelydiffuses though not so easily as in porous membranes to thereby give apoor battery current efficiency. The membrane disclosed in PatentLiterature 3 also has the similar problem as in the above, and isinferior also in the oxidation-resistant durability.

The battery disclosed in Patent Literature 4 is yet insufficient in thecurrent efficiency, inferior also in the oxidative deteriorationresistance in a sulfuric acid electrolyte solution over a long period,and insufficient also in the durability. The Patent Literature, althoughdisclosing a battery using a PTFE-based ion-exchange membrane as acomparative example, states that the current efficiency is 64.8 to 78.6and insufficient.

The battery disclosed in Patent Literature 5 also cannot solve thesimilar problem as in the above, and a problem thereof is that thelarge-size facility is resultantly high in price.

Patent Literature 6 states that the membrane disclosed therein increasesin the internal resistance unless the thickness of a coated membrane ismade extremely thin (several micrometers). No devices to improve the ionpermselectivity are described at all.

The battery disclosed in Patent Literature 7, since using apolysulfone-based separation membrane, is not sufficient in the ionpermselectivity and the oxidative deterioration resistance of theseparation membrane, and is not sufficient in the electric resistance,the current efficiency, and the durability of the battery.

The battery disclosed in Patent Literature 8 is insufficient in thecurrent efficiency, and has a problem with the long-term usage becauseof oxidative deterioration.

A problem of the membrane disclosed in Patent Literature 9 is that theelectric resistance is high.

The result shown in an Example of Patent Literature 10 cannot be said toexhibit a sufficiently low internal resistance (electric resistance) ofthe membrane, and has the problem of the oxidative deteriorationresistance in the long-term usage.

Electrolyte membranes (separation membranes) for conventionalvanadium-type redox flow batteries are used for the purpose ofsuppressing the diffusion, migration, and permeation of active substanceions to counter electrodes (cells), and allowing protons (H⁺) toselectively permeate along with the operation of charge and discharge asthe purpose, in each of a cell (negative electrode side) in which ionsof a low-valent group of vanadium ions, which are active substances ofelectrolyte solutions of both electrodes, hold a large majority, and acell (positive electrode side) in which ions of a high-valent group ofthe vanadium ions hold a large majority. However, the performance cannotbe said to be sufficient at present.

As a membrane base material composed mainly of a hydrocarbon-basedresin, there are used a porous membrane which only simply separateselectrolyte solutions containing electrolytes as principal parts of bothcells and exhibits no ion permselectivity, a (nonporous) hydrophilicmembrane base material exhibiting no ion permselectivity, a porousmembrane having a hydrophilic membrane base material embedded therein orcovered thereon, and the like. There are also used as a separationmembrane a so-called cation-exchange membrane in which the membraneitself has various types of anion groups, or a composite membrane inwhich a cation-exchange resin is covered on or embedded in pores of aporous membrane base material, an anion-exchange membrane in which themembrane itself similarly has cation groups, or a composite membrane inwhich an anion-exchange resin is similarly covered on or embedded in aporous membrane base material, a membrane of a laminate type of both,and the like; and studies making the most of respective features arebeing carried out.

No ion-exchange resin separation membrane as the separation membrane hasbeen developed so far which sufficiently satisfies two contraryperformances of the electric resistance (depending mainly on the protonpermeability) and the permeability inhibition of metal ions (polyvalentcations), which are active substances as the principal parts, andfurther no ion-exchange resin separation membrane has been developed sofar which satisfies, in addition to the above two performances, theoxidative deterioration resistance (hydroxy radical resistance) over along period. Also for fluorine-based ion-exchange resins, no sufficientstudies of devices have been made on mutually contradictory propertiesof the excellent proton (H⁺) permeability and the inhibition of theactive substance ion permeation; and no redox flow battery and noelectrolyte membrane therefor have been developed which sufficientlysatisfy a low electric resistance, a high current efficiency, theoxidative deterioration resistance (hydroxy radical resistance), and thelike.

In consideration of the above-mentioned situation, it is an object ofthe present invention to provide a redox flow secondary battery beinglow in the electric resistance and excellent in the current efficiency,and further having the durability; and it is an object of the presentinvention to provide an electrolyte membrane for a redox flow secondarybattery having the excellent ion permselectivity capable of suppressingthe active substance ion permeability without deteriorating the proton(H⁺) permeability, and further having the oxidative deteriorationresistance (hydroxy radical resistance) as well.

Solution to Problem

As a result of exhaustive studies to solve the above-mentioned problems,the present inventors have found that an electrolyte membrane can beprovided which contains a fluorine-based polyelectrolyte polymer havinga specific structure, has an excellent ion permselectivity by regulatingan amount of the fluorine ions eluted in a specific range in animmersion test using a Fenton's reagent solution, and is furtherexcellent also in the oxidative deterioration resistance (hydroxyradical resistance). It has been further found that a redox flowsecondary battery can be provided which is low in the electricresistance, excellent in the current efficiency, and further excellentin the durability by using the above electrolyte membrane as theseparation membrane, and these findings have led to the completion ofthe present invention.

That is, the present invention is as follows.

[1]

A redox flow secondary battery comprising an electrolytic bathcomprising:

a positive electrode cell chamber comprising a positive electrodecomposed of a carbon electrode;

a negative electrode cell chamber comprising a negative electrodecomposed of a carbon electrode; and

an electrolyte membrane as a separation membrane to separate thepositive electrode cell chamber and the negative electrode cell chamber,

wherein the positive electrode cell chamber comprises a positiveelectrode electrolyte solution comprising a positive electrode activesubstance; and the negative electrode cell chamber comprises a negativeelectrode electrolyte solution comprising a negative electrode activesubstance;

wherein the redox flow secondary battery charges and discharges based onchanges in valences of the positive electrode active substance and thenegative electrode active substance in the electrolyte solutions;

wherein the electrolyte membrane comprises an ion-exchange resincomposition comprising a fluorine-based polyelectrolyte polymer having astructure represented by the following formula (1):

—[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1)

wherein X¹, X², and X³ each independently represent one or more selectedfrom the group consisting of halogen atoms and perfluoroalkyl groupshaving 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZwherein Z represents a hydrogen atom, an alkali metal atom, an alkalineearth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄)wherein R₁, R₂, R₃, and R₄ each independently represent one or moreselected from the group consisting of alkyl groups and arene groups,when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² eachindependently represent one or more selected from the group consistingof halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groupshaving 1 to 10 carbon atoms; and a and g represent numbers satisfying0≦a<1, 0<g≦1, and a+g=1, b represents an integer of 0 to 8, c represents0 or 1, and d, e, and f each independently represent an integer of 0 to6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein in a test in which 0.1 g of the fluorine-based polyelectrolytepolymer is immersed in 50 g of a Fenton's reagent solution containing a3% hydrogen peroxide solution and 200 ppm of divalent iron ions at 40°C. for 16 hours, an amount of fluorine ions eluted detected in asolution is 0.03% or smaller of a whole amount of fluorine in a immersedpolymer.

[2]

The redox flow secondary battery according to above [1], wherein in thetest in which 0.1 g of the fluorine-based polyelectrolyte polymer isimmersed in 50 g of the Fenton's reagent solution containing the 3%hydrogen peroxide solution and 200 ppm of divalent iron ions at 40° C.for 16 hours, the amount of fluorine ions eluted detected in thesolution is 0.002% or smaller of the whole amount of fluorine in theimmersed polymer.

[3]

A redox flow secondary battery comprising an electrolytic bathcomprising:

a positive electrode cell chamber comprising a positive electrodecomposed of a carbon electrode;

a negative electrode cell chamber comprising a negative electrodecomposed of a carbon electrode; and

an electrolyte membrane as a separation membrane to separate thepositive electrode cell chamber and the negative electrode cell chamber,

wherein the positive electrode cell chamber comprises a positiveelectrode electrolyte solution comprising a positive electrode activesubstance; and the negative electrode cell chamber comprises a negativeelectrode electrolyte solution comprising a negative electrode activesubstance;

wherein the redox flow secondary battery charges and discharges based onchanges in valences of the positive electrode active substance and thenegative electrode active substance in the electrolyte solutions;

wherein the electrolyte membrane comprises an ion-exchange resincomposition comprising a fluorine-based polyelectrolyte polymer having astructure represented by the following formula (1):

—[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1)

wherein X¹, X², and X³ each independently represent one or more selectedfrom the group consisting of halogen atoms and perfluoroalkyl groupshaving 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZwherein Z represents a hydrogen atom, an alkali metal atom, an alkalineearth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄)wherein R₁, R₂, R₃, and R₄ each independently represent one or moreselected from the group consisting of alkyl groups and arene groups,when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² eachindependently represent one or more selected from the group consistingof halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groupshaving 1 to 10 carbon atoms; and a and g represent numbers satisfying0≦a<1, 0<g≦1, and a+g=1, b represents an integer of 0 to 8, c represents0 or 1, and d, e, and f each independently represent an integer of 0 to6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein the ion-exchange resin composition comprises 0.1 to 20 parts bymass of a polyphenylene ether resin and/or a polyphenylene sulfide resinwith respect to 100 parts by mass of the fluorine-based polyelectrolytepolymer.

[4]

A redox flow secondary battery comprising an electrolytic bathcomprising:

a positive electrode cell chamber comprising a positive electrodecomposed of a carbon electrode;

a negative electrode cell chamber comprising a negative electrodecomposed of a carbon electrode; and

an electrolyte membrane as a separation membrane to separate thepositive electrode cell chamber and the negative electrode cell chamber,

wherein the positive electrode cell chamber comprises a positiveelectrode electrolyte solution comprising a positive electrode activesubstance; and the negative electrode cell chamber comprises a negativeelectrode electrolyte solution comprising a negative electrode activesubstance;

wherein the redox flow secondary battery charges and discharges based onchanges in valences of the positive electrode active substance and thenegative electrode active substance in the electrolyte solutions;

wherein the electrolyte membrane comprises an ion-exchange resincomposition comprising a fluorine-based polyelectrolyte polymer having astructure represented by the following formula (1):

—[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1)

wherein X¹, X², and X³ each independently represent one or more selectedfrom the group consisting of halogen atoms and perfluoroalkyl groupshaving 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZwherein Z represents a hydrogen atom, an alkali metal atom, an alkalineearth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄)wherein R₁, R₂, R₃, and R₄ each independently represent one or moreselected from the group consisting of alkyl groups and arene groups,when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² eachindependently represent one or more selected from the group consistingof halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groupshaving 1 to 10 carbon atoms; and a and g represent numbers satisfying0≦a<1, 0<g≦1, and a+g=1, b represents an integer of 0 to 8, c represents0 or 1, and d, e, and f each independently represent an integer of 0 to6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein the ion-exchange resin composition comprises a Ce-basedadditive.

[5]

A redox flow secondary battery comprising an electrolytic bathcomprising:

a positive electrode cell chamber comprising a positive electrodecomposed of a carbon electrode;

a negative electrode cell chamber comprising a negative electrodecomposed of a carbon electrode; and

an electrolyte membrane as a separation membrane to separate thepositive electrode cell chamber and the negative electrode cell chamber,

wherein the positive electrode cell chamber comprises a positiveelectrode electrolyte solution comprising a positive electrode activesubstance; and the negative electrode cell chamber comprises a negativeelectrode electrolyte solution comprising a negative electrode activesubstance;

wherein the redox flow secondary battery charges and discharges based onchanges in valences of the positive electrode active substance and thenegative electrode active substance in the electrolyte solutions;

wherein the electrolyte membrane comprises an ion-exchange resincomposition comprising a fluorine-based polyelectrolyte polymer having astructure represented by the following formula (1):

—[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1)

wherein X¹, X², and X³ each independently represent one or more selectedfrom the group consisting of halogen atoms and perfluoroalkyl groupshaving 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZwherein Z represents a hydrogen atom, an alkali metal atom, an alkalineearth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄)wherein R₁, R₂, R₃, and R₄ each independently represent one or moreselected from the group consisting of alkyl groups and arene groups,when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² eachindependently represent one or more selected from the group consistingof halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groupshaving 1 to 10 carbon atoms; and a and g represent numbers satisfying0≦a<1, 0<g≦1, and a+g=1, b represents an integer of 0 to 8, c represents0 or 1, and d, e, and f each independently represent an integer of 0 to6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein the ion-exchange resin composition comprises a Co-based and/or aMn-based additive.

[6]

The redox flow secondary battery according to any of above [1] to [5],wherein sulfuric acid electrolyte solutions comprising vanadium are usedas the positive electrode electrolyte solution and the negativeelectrode electrolyte solution.

[7]

The redox flow secondary battery according to any of above [1] to [6],wherein the fluorine-based polyelectrolyte polymer is aperfluorocarbonsulfonic acid resin (PFSA) having a structure representedby the following formula (2):

—[CF₂—CF₂]_(a)—[CF₂—CF((—O—(CF₂)_(m)—SO₃H)]_(g)—  (2)

wherein a and g represent numbers satisfying 0≦a<1, 0<g≦1, and a+g=1;and m represents an integer of 1 to 6.[8]

The redox flow secondary battery according to any of above [1] to [7],wherein the fluorine-based polyelectrolyte polymer has an equivalentweight EW (dry mass in grams per equivalent of ion-exchange groups) of300 to 1,300 g/eq; and the electrolyte membrane has an equilibriummoisture content of 5 to 80% by mass.

[9]

An electrolyte membrane for a redox flow secondary battery, comprisingan ion-exchange resin composition comprising a fluorine-basedpolyelectrolyte polymer having a structure represented by the followingformula (1):

—[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1)

wherein X¹, X², and X³ each independently represent one or more selectedfrom the group consisting of halogen atoms and perfluoroalkyl groupshaving 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZwherein Z represents a hydrogen atom, an alkali metal atom, an alkalineearth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄)wherein R₁, R₂, R₃, and R₄ each independently represent one or moreselected from the group consisting of alkyl groups and arene groups,when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² eachindependently represent one or more selected from the group consistingof halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groupshaving 1 to 10 carbon atoms; and a and g represent numbers satisfying0≦a<1, 0<g≦1, and a+g=1, b represents an integer of 0 to 8, c represents0 or 1, and d, e, and f each independently represent an integer of 0 to6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein in a test in which 0.1 g of the fluorine-based polyelectrolytepolymer is immersed in 50 g of a Fenton's reagent solution containing a3% hydrogen peroxide solution and 200 ppm of divalent iron ions at 40°C. for 16 hours, an amount of fluorine ions eluted detected in asolution is 0.03% or smaller of a whole amount of fluorine in a immersedpolymer.

[10]

The electrolyte membrane for a redox flow secondary battery according toabove [9], wherein in the test in which 0.1 g of the fluorine-basedpolyelectrolyte polymer is immersed in 50 g of the Fenton's reagentsolution containing the 3% hydrogen peroxide solution and 200 ppm ofdivalent iron ions at 40° C. for 16 hours, the amount of fluorine ionseluted detected in the solution is 0.002% or smaller of the whole amountof fluorine in the immersed polymer.

[11]

An electrolyte membrane for a redox flow secondary battery, comprisingan ion-exchange resin composition comprising a fluorine-basedpolyelectrolyte polymer having a structure represented by the followingformula (1):

—[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1)

wherein X¹, X², and X³ each independently represent one or more selectedfrom the group consisting of halogen atoms and perfluoroalkyl groupshaving 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZwherein Z represents a hydrogen atom, an alkali metal atom, an alkalineearth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄)wherein R₁, R₂, R₃, and R₄ each independently represent one or moreselected from the group consisting of alkyl groups and arene groups,when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² eachindependently represent one or more selected from the group consistingof halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groupshaving 1 to 10 carbon atoms; and a and g represent numbers satisfying0≦a<1, 0<g≦1, and a+g=1, b represents an integer of 0 to 8, c represents0 or 1, and d, e, and f each independently represent an integer of 0 to6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein the ion-exchange resin composition comprises 0.1 to 20 parts bymass of a polyphenylene ether resin and/or a polyphenylene sulfide resinwith respect to 100 parts by mass of the fluorine-based polyelectrolytepolymer.

[12]

An electrolyte membrane for a redox flow secondary battery, comprisingan ion-exchange resin composition comprising a fluorine-basedpolyelectrolyte polymer having a structure represented by the followingformula (1):

—[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1)

wherein X¹, X², and X³ each independently represent one or more selectedfrom the group consisting of halogen atoms and perfluoroalkyl groupshaving 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZwherein Z represents a hydrogen atom, an alkali metal atom, an alkalineearth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄)wherein R₁, R₂, R₃, and R₄ each independently represent one or moreselected from the group consisting of alkyl groups and arene groups,when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² eachindependently represent one or more selected from the group consistingof halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groupshaving 1 to 10 carbon atoms; and a and g represent numbers satisfying0≦a<1, 0<g≦1, and a+g=1, b represents an integer of 0 to 8, c represents0 or 1, and d, e, and f each independently represent an integer of 0 to6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein the ion-exchange resin composition comprises a Ce-basedadditive.

[13]

An electrolyte membrane for a redox flow secondary battery, comprisingan ion-exchange resin composition comprising a fluorine-basedpolyelectrolyte polymer having a structure represented by the followingformula (1):

—[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1)

wherein X¹, X², and X³ each independently represent one or more selectedfrom the group consisting of halogen atoms and perfluoroalkyl groupshaving 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZwherein Z represents a hydrogen atom, an alkali metal atom, an alkalineearth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄)wherein R₁, R₂, R₃, and R₄ each independently represent one or moreselected from the group consisting of alkyl groups and arene groups,when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² eachindependently represent one or more selected from the group consistingof halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groupshaving 1 to 10 carbon atoms; and a and g represent numbers satisfying0≦a<1, 0<g≦1, and a+g=1, b represents an integer of 0 to 8, c represents0 or 1, and d, e, and f each independently represent an integer of 0 to6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein the ion-exchange resin composition comprises a Co-based and/or aMn-based additive.

[14]

The electrolyte membrane for a redox flow secondary battery according toany of above [9] to [13], wherein the fluorine-based polyelectrolytepolymer is a perfluorocarbonsulfonic acid resin (PFSA) having astructure represented by the following formula (2):

—[CF₂—CF₂]_(a)—[CF₂—CF((—O—(CF₂)_(m)—SO₃H)]_(g)—  (2)

wherein a and g represent numbers satisfying 0≦a<1, 0<g≦1, and a+g=1;and m represents an integer of 1 to 6.[15]

The electrolyte membrane for a redox flow secondary battery according toany of above [9] to [14], wherein the fluorine-based polyelectrolytepolymer has an equivalent weight EW (dry mass in grams per equivalent ofion-exchange groups) of 300 to 1,300 g/eq; and the electrolyte membranehas an equilibrium moisture content of 5 to 80% by mass.

[16]

The electrolyte membrane for a redox flow secondary battery according toany of above [9] to [15], wherein the electrolyte membrane is subjectedto a heat treatment at 130 to 200° C. for 1 to 60 min.

Advantageous Effects of Invention

The redox flow secondary battery according to the present invention islow in the electric resistance and high in the current efficiency, andcan further suppress the elimination of ion groups, the collapsephenomenon of a polyelectrolyte, and the like as compared with redoxflow secondary batteries using a hydrocarbon-based electrolyte as aseparation membrane, and is excellent in the durability.

Since the electrolyte membrane for a redox flow secondary batteryaccording to the present invention has excellent ion permselectivity, isexcellent in the high proton (H⁺) permeability and the permeationinhibition of active substance ions in electrolyte solutions, and isfurther excellent in the oxidative deterioration resistance (hydroxyradical resistance) over a long period, the use of the electrolytemembrane as a separation membrane of a redox flow secondary battery canprovide the redox flow secondary battery low in the cell electricresistance and high in the current efficiency; and since the electrolytemembrane exhibits a high oxidative deterioration prevention effect tohydroxy radicals generated in electrolyte solution cells in a systemover a long period, the electrolyte membrane can suppress theelimination of ion groups, the collapse phenomenon of thepolyelectrolyte, and the like, which are caused in the case of usingusual hydrocarbon-based electrolytes.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 shows an example of a schematic diagram of a redox flow secondarybattery in the present embodiments.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments to carry out the present invention(hereinafter, referred to as “present embodiments”) will be described indetail. The present invention is not limited to the following presentembodiments. Hereinafter, the present embodiments 1 to 4 will bedescribed in order.

The Present Embodiment 1 Redox Flow Secondary Battery

A redox flow secondary battery in the present embodiment 1, comprisingan electrolytic bath comprising:

a positive electrode cell chamber comprising a positive electrodecomposed of a carbon electrode;

a negative electrode cell chamber comprising a negative electrodecomposed of a carbon electrode; and

an electrolyte membrane as a separation membrane to separate thepositive electrode cell chamber and the negative electrode cell chamber,

wherein the positive electrode cell chamber comprises a positiveelectrode electrolyte solution comprising a positive electrode activesubstance; and the negative electrode cell chamber comprises a negativeelectrode electrolyte solution comprising a negative electrode activesubstance;

wherein the redox flow secondary battery charges and discharges based onchanges in valences of the positive electrode active substance and thenegative electrode active substance in the electrolyte solutions;

wherein the electrolyte membrane comprises an ion-exchange resincomposition comprising a fluorine-based polyelectrolyte polymer having astructure represented by the following formula (1):

—[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1)

wherein X¹, X², and X³ each independently represent one or more selectedfrom the group consisting of halogen atoms and perfluoroalkyl groupshaving 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZwherein Z represents a hydrogen atom, an alkali metal atom, an alkalineearth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄)wherein R₁, R₂, R₃, and R₄ each independently represent one or moreselected from the group consisting of alkyl groups and arene groups,when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² eachindependently represent one or more selected from the group consistingof halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groupshaving 1 to 10 carbon atoms; and a and g represent numbers satisfying0≦a<1, 0<g≦1, and a+g=1, b represents an integer of 0 to 8, c represents0 or 1, and d, e, and f each independently represent an integer of 0 to6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein in a test in which 0.1 g of the fluorine-based polyelectrolytepolymer is immersed in 50 g of a Fenton's reagent solution containing a3% hydrogen peroxide solution and 200 ppm of divalent iron ions at 40°C. for 16 hours, an amount of fluorine ions eluted detected in thesolution is 0.03% or smaller of the whole amount of fluorine in theimmersed polymer.

FIG. 1 shows an example of a schematic diagram of a redox flow secondarybattery in the present embodiment 1. A redox flow secondary battery 10in the present embodiment 1 has an electrolytic bath 6 which comprises apositive electrode cell chamber 2 comprising a positive electrode 1composed of a carbon electrode, a negative electrode cell chamber 4comprising a negative electrode 3 composed of a carbon electrode, and anelectrolyte membrane 5 as a separation membrane to separate the positiveelectrode cell chamber 2 and the negative electrode cell chamber 4,wherein the positive electrode cell chamber 2 contains a positiveelectrode electrolyte solution comprising a positive electrode activesubstance; and the negative electrode cell chamber 4 contains a negativeelectrode electrolyte solution comprising a negative electrode activesubstance. The positive electrode electrolyte solution and the negativeelectrode electrolyte solution comprising the active substances are, forexample, stored in a positive electrode electrolyte solution tank 7 anda negative electrode electrolyte solution tank 8, and fed to respectivecell chambers (arrows A and B) by pumps or the like. The currentgenerated by the redox flow secondary battery may be converted fromdirect current to alternating current through an AC/DC converter 9.

The redox flow secondary battery in the present embodiment 1 has astructure in which each of liquid-permeable porous current collectorelectrodes (for the negative electrode and for the positive electrode)is disposed on either side of the separation membrane, and these areheld by pressing; one side partitioned by the separation membrane ismade the positive electrode cell chamber and the other side is made thenegative electrode cell chamber; and the thicknesses of both the cellchambers are secured by spacers.

In the case of a vanadium-type redox flow secondary battery, the chargeand discharge of the battery is carried out by circulating the positiveelectrode electrolyte solution composed of a sulfuric acid electrolytesolution comprising tetravalent vanadium (e) and pentavalent vanadium(V⁵⁺) to the positive electrode cell chamber, and circulating thenegative electrode electrolyte solution comprising trivalent vanadium(V³⁺) and divalent vanadium (V²⁺) to the negative electrode cellchamber. In the charge time therein, in the positive electrode cellchamber, vanadium ions release electrons to thereby oxidize V⁴⁺ to V⁵⁺;and in the negative electrode cell chamber, electrons having returnedthrough an external circuit reduce V³⁺ to V²⁺. In the oxidation andreduction reactions, in the positive electrode cell chamber, protons (W)become excessive; by contrast, in the negative electrode cell chamber,protons (H⁺) become insufficient. The excessive protons in the positiveelectrode cell chamber selectively migrate to the negative electrodechamber through the separation membrane to thereby hold the electricneutrality. In the discharge time, a reaction reverse theretoprogresses. The battery efficiency (%) at this time is represented by aratio (%) obtained by dividing a discharge electric energy by a chargeelectric energy; and both the electric energies depend on the internalresistance of the battery cells, the ion permselectivity of theseparation membrane, and the current losses of others. Since thereduction of the internal resistance improves the voltage efficiency,and the improvement of the ion permselectivity and the reduction of thecurrent losses of others improve the current efficiency, these factorsbecome important indices in the redox flow secondary battery.

[Electrolyte Membrane for a Redox Flow Secondary Battery]

The electrolyte membrane for a redox flow secondary battery in thepresent embodiment 1 has a specific structure, and comprises anion-exchange resin composition comprising a fluorine-basedpolyelectrolyte polymer a part of whose molecular chain terminals isfluorinated.

<Ion-Exchange Resin Composition>

In the present embodiment 1, the ion-exchange resin compositioncomprises a fluorine-based polyelectrolyte polymer having a structurerepresented by the above formula (1).

(Fluorine-Based Polyelectrolyte Polymer)

In the present embodiment 1, the fluorine-based polyelectrolyte polymerhas a structure represented by the following formula (1):

—[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1)

wherein X¹, X², and X³ each independently represent one or more selectedfrom the group consisting of halogen atoms and perfluoroalkyl groupshaving 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZwherein Z represents a hydrogen atom, an alkali metal atom, an alkalineearth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄)wherein R₁, R₂, R₃, and R₄ each independently represent one or moreselected from the group consisting of alkyl groups and arene groups,when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² eachindependently represent one or more selected from the group consistingof halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groupshaving 1 to 10 carbon atoms; and a and g represent numbers satisfying0≦a<1, 0<g≦1, and a+g=1, b represents an integer of 0 to 8, c represents0 or 1, and d, e, and f each independently represent an integer of 0 to6 (with the proviso that d, e, and f are not 0 at the same time).

X¹, X², and X³ each independently represent one or more selected fromthe group consisting of halogen atoms and perfluoroalkyl groups having 1to 3 carbon atoms. Here, the halogen atoms include a fluorine atom, achlorine atom, a bromine atom, and an iodine atom. X¹, X² and X³, fromthe viewpoint of the chemical stability including the oxidativedeterioration resistance of the polymer, are preferably each a fluorineatom or a perfluoroalkyl group having 1 to 3 carbon atoms.

X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZ. Hereinafter, X⁴ is alsoreferred to as an “ion-exchange group.” Z represents a hydrogen atom, analkali metal atom, an alkaline earth metal atom, or amines (NH₄, NH₃R₁,NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄). Here, the alkali metal atom is notespecially limited, and includes a lithium atom, a sodium atom, and apotassium atom. The alkaline earth metal atom is not especially limited,and includes a calcium atom and a magnesium atom. R₁, R₇, R₃, and R₄each independently represent one or more selected from the groupconsisting of alkyl groups and arene groups. Here, in the case where X⁴is PO₃Z₂, Z may be identical or different. X⁴, from the viewpoint of thechemical stability including the oxidative deterioration resistance ofthe polymer, is preferably SO₃Z.

R¹ and R² each independently represent one or more selected from thegroup consisting of halogen atoms and perfluoroalkyl groups andfluorochloroalkyl groups having 1 to 10 carbon atoms. Here, the halogenatoms include a fluorine atom, a chlorine atom, a bromine atom, and aniodine atom.

a and g represent numbers satisfying 0≦a<1, 0<g≦1, and a+g=1. brepresents an integer of 0 to 8. c represents 0 or 1. d, e, and f eachindependently represent an integer of 0 to 6. Here, d, e, and f are not0 at the same time.

The fluorine-based polyelectrolyte polymer in the present embodiment 1is preferably a perfluorocarbonsulfonic acid resin (hereinafter, alsoreferred to as “PFSA resin”) because of giving a tendency of making theadvantage of the present invention more remarkable. The PFSA resin inthe present embodiment is a resin in which perfluorocarbons as sidechains are bonded to the main chain composed of a PTFE skeleton chain,and one or two or more sulfonic acid groups (as the case may be, a partof the groups may be a form of a salt) are bonded to the each sidechain.

The PFSA resin preferably comprises a repeating unit represented by—(CF₂—CF₂)— and a repeating unit derived from a compound represented bythe following formula (3) or (4), and is further preferably composed ofa repeating unit represented by —(CF₂—CF₂)— and a repeating unit derivedfrom a compound represented by the formula (3) or (4).

Formula (3): CF₂═CF(—O—(CF₂CFXO)_(n)— [A]) wherein X represents F or aperfluoroalkyl group having 1 to 3 carbon atoms; n represents an integerof 0 to 5; and [A] is (CF₂)_(m)—SO₃H wherein m represents an integer of1 to 6, here, n and m are not 0 at the same time, orFormula (4): CF₂═CF—O—(CF₂)_(P)—CFX(—O—(CF₂)_(K)SO₃H) orCF₂═CF—O—(CF₂)_(P)—CFX(—(CF₂)_(L)—O—(CF₂)_(m)—SO₃H) wherein X representsa perfluoroalkyl group having 1 to 3 carbon atoms; and P represents aninteger of 0 to 12, K represents an integer of 1 to 5, L represents aninteger of 1 to 5, and m represents an integer of 0 to 6, here, K and Lmay be identical or different, and P, K, and L are not 0 at the sametime.

The PFSA resin is a copolymer comprising a repeating unit represented by—(CF₂—CF₂)— and a repeating unit represented by—(CF₂—CF(—O—(CF₂CFXO)_(n)—(CF₂)_(m)—SO₃H))— wherein X represents F orCF₃; and n represents an integer of 0 to 5, and m represents an integerof 0 to 12, here, n and m are not 0 at the same time, and is morepreferably a copolymer necessarily comprising a repeating unitrepresented by —(CF₂—CF(—O—(CF₂CFXO)_(n)—(CF₂)_(m)—SO₃H))— wherein Xrepresents CF₃; and n represents 0 or 1, and m represents an integer of0 to 12, here, n and m are not 0 at the same time. The case where thePFSA resin is a copolymer having the above structure and has apredetermined equivalent weight EW has such tendencies that an obtainedelectrolyte membrane exhibits sufficient hydrophilicity, and theresistance to radical species generated by oxidative deteriorationbecomes high.

The case where the PFSA resin comprises the repeating unit of—(CF₂—CF(—O—(CF₂CFXO)_(n)— (CF₂)_(m)—SO₃H))— wherein n is 0 and m is aninteger of 1 to 6, or both the repeating units of—CF₂—CF(—O—(CF₂)_(P)—CFX(—O—(CF₂)_(K)—SO₃H)— and—CF₂—CF(—O—(CF₂)_(P)—CFX(—(CF₂)_(L)—O—(CF₂)_(m)—SO₃H)— represented bythe formula (4) has further such tendencies that the equivalent weightEW becomes low and the hydrophilicity of an obtained electrolytemembrane becomes high.

In the copolymer, of Nafion (registered trademark of Du Pont K. K.)which is a fluorine-based resin used in the conventional technology,containing a repeating unit represented by —(CF₂—CF₂)— and a repeatingunit of —(CF₂—CF(—O—(CF₂CFXO)_(n)—(CF₂)_(m)—SO₃H))—, it is known thatX═CF₃, n=1 and m=2; and the EW described later is 893 to 1,030.

It has been found as a result of studies by the present inventors thatin the case where a PFSA resin is used as an electrolyte membrane for aredox flow secondary battery, the PFSA resin comprising the repeatingunit represented by —(CF₂—CF(—O—(CF₂CFXO)_(n)—(CF₂)_(m)—SO₃H))— whereinn is 0 and m is an integer of 1 to 6, or both the repeating units of—CF₂—CF(—O—(CF₂)_(P)—CFX(—O—(CF₂)_(K)—SO₃H)— and—CF₂—CF(—O—(CF₂)₂—CFX(—(CF₂)_(L)—O—(CF₂)_(m)—SO₃H)— represented by theformula (4) has such tendencies that the hydrophilicity and the ionpermselectivity are excellent, and the electric resistance of anobtained redox flow secondary battery is low and the current efficiencythereof is improved, as compared with the above Nafion.

The fluorine-based polyelectrolyte polymer represented by the formula(1) in the present embodiment 1 is preferably a PFSA resin having astructure represented by the following formula (2) because of giving atendency of making the advantage of the present invention moreremarkable.

—[CF₂CF₂]_(a)—[CF₂—CF((—O—(CF₂)_(m)—SO₃H)]_(g)—  (2)

wherein a and g represent numbers satisfying 0≦a<1, 0<g≦1, and a+g=1; mrepresents an integer of 1 to 6.

The fluorine-based polyelectrolyte polymer represented by the aboveformula (1) and the PFSA resin having a structure represented by theabove formula (2) in the present embodiment 1, respectively, are notespecially limited as long as having the structures represented by theabove formula (1) and the above formula (2), and may comprise otherstructures.

The fluorine-based polyelectrolyte polymer represented by the aboveformula (1) and the PFSA resin having a structure represented by theabove formula (2) in the present embodiment 1 may be those in which apart of ion-exchange groups is subjected to intermolecular direct orindirect partial crosslinking reaction. The partial crosslinking ispreferable from the viewpoint of being able to control the solubilityand the excessive swell.

For example, even if the EW of a fluorine-based polyelectrolyte polymeris about 280, by carrying out the above partial crosslinking, thesolubility of the fluorine-based polyelectrolyte polymer to water can bereduced (the water resistance can be improved).

Also in the case where a fluorine-based polyelectrolyte polymer is in alow melt flow region (polymer region), the above partial crosslinkingcan increase intermolecular entanglement and reduce the solubility andthe excessive swell.

Examples of the partial crosslinking reaction include a reaction of anion-exchange group with a functional group or the main chain of anothermolecule, a reaction of ion-exchange groups, and a crosslinking reaction(covalent bond) through an oxidation-resistant low molecular compound,oligomer, polymeric substance, or the like, and as the case may be, areaction with a substance to form a salt (including an ionic bond with aSO₃H group). Examples of the oxidation-resistant low molecular compound,oligomer, and polymeric substance include polyhydric alcohols andorganic diamines.

(Equivalent Weight EW of a Fluorine-Based Polyelectrolyte Polymer)

The equivalent weight EW (dry mass in grams of a fluorine-basedpolyelectrolyte polymer per equivalent weight of an ion-exchange group)of the fluorine-based polyelectrolyte polymer in the present embodiment1 is preferably 300 to 1,300 (g/eq), more preferably 350 to 1,000(g/eq), still more preferably 400 to 900 (g/eq), and especiallypreferably 450 to 750 (g/eq).

In a fluorine-based polyelectrolyte polymer having a structure of theabove formula (1), by regulating the equivalent weight EW thereof in theabove range, an ion-exchange resin composition containing the polymercan be imparted with excellent hydrophilicity; and an electrolytemembrane obtained by using the resin composition results in having alower electric resistance and a higher hydrophilicity, and having alarge number of smaller clusters (minute moieties where ion-exchangegroups coordinate and/or adsorb water molecules), and gives such atendency that the oxidation resistance (hydroxy radical resistance) andthe ion permselectivity are more improved.

The equivalent weight EW of a fluorine-based polyelectrolyte polymer ispreferably 300 or higher from the viewpoint of the hydrophilicity andthe water resistance of the membrane; and that is preferably 1,300 orlower from the viewpoint of the hydrophilicity and the electricresistance of the membrane.

The equivalent weight EW of a fluorine-based polyelectrolyte polymer canbe measured by replacing the fluorine-based polyelectrolyte polymer by asalt, and back-titrating the solution with an alkali solution.

The equivalent weight EW can be regulated by selecting copolymerizationratios of fluorine-based monomers as raw materials of a fluorine-basedpolyelectrolyte polymer, kinds of the monomers, and the like.

(Method for Producing a Fluorine-Based Polyelectrolyte Polymer)

A fluorine-based polyelectrolyte polymer in the present embodiment 1 canbe obtained, for example, by producing a precursor of a polyelectrolytepolymer (hereinafter, also referred to as “resin precursor”), andthereafter subjecting the precursor to a hydrolysis treatment.

A PFSA resin can be obtained, for example, by hydrolyzing a PFSA resinprecursor composed of a copolymer of a fluorinated vinyl ether compoundrepresented by the following general formula (5) or (6) with afluorinated olefin monomer represented by the following general formula(7).

CF₂═CF—O—(CF₂CFXO)_(n)-A  Formula (5):

wherein X represents F or a perfluoroalkyl group having 1 to 3 carbonatoms; n represents an integer of 0 to 5; and A represents (CF₂)_(m)—W,and W represents a functional group capable of being converted to SO₃Hby hydrolysis.

CF₂═CF—O—(CF₂)_(P)—CF((—O—(CF₂)_(K)—W) orCF₂═CF—O—(CF₂)_(P)—CF(—(CF₂)_(L)—O—(CF₂)_(m)—W)  Formula (6):

wherein p represents an integer of 0 to 12, and m represents an integerof 0 to 6, here, n and m are not 0 at the same time; K represents aninteger of 1 to 5; L represents an integer of 1 to 5, here, n and L or Kare not 0 at the same time; and W represents a functional group capableof being converted to SO₃H by hydrolysis.

CF₂═CFZ  Formula (7):

wherein Z represents H, Cl, F, a perfluoroalkyl group having 1 to 3carbon atoms, or a cyclic perfluoroalkyl group which may contain oxygen.

W denoting a functional group capable of being converted to SO₃H byhydrolysis in the above formula (5) is not especially limited, but ispreferably SO₂F, SO₂Cl, or SO₂Br. Further in the above formulae, X═CF₃,W=SO₂F, and Z═F are more preferable. Particularly, n=0, m=an integer of1 to 6, X═CF₃, W=SO₂F, and Z═F are especially preferable because ofgiving tendencies of providing high hydrophilicity and a solution havinga high resin concentration.

The above resin precursor in the present embodiment 1 can be synthesizedby well-known means. The resin precursor can be produced, for example,by polymerizing a fluorinated vinyl compound having a group(ion-exchange group precursor group) capable of being converted to anion-exchange group (X⁴ in the formula (1)) by hydrolysis or the like inthe presence of a radical generator such as a peroxide or the like, witha fluorinated olefin such as tetrafluoroethylene (TFE). Thepolymerization method is not especially limited, and usable methodsthereof include a method (solution polymerization) of filling anddissolving and reacting the fluorinated vinyl compound or the like and agas of the fluorinated olefin in a polymerization solvent such as afluorine-containing hydrocarbon, to thereby carry out thepolymerization, a method (bulk polymerization) of carrying out thepolymerization by using the fluorinated vinyl compound itself as apolymerization solvent without using any solvent such as afluorine-containing hydrocarbon, a method (emulsion polymerization) offilling and reacting the fluorinated vinyl compound and a gas of thefluorinated olefin by using an aqueous solution of a surfactant as amedium, to thereby carry out the polymerization, a method (emulsionpolymerization) of filling and emulsifying and reacting the fluorinatedvinyl compound and a gas of the fluorinated olefin in an aqueoussolution of a surfactant and an emulsifying aid such as an alcohol tothereby carry out the polymerization, and a method (suspensionpolymerization) of filling and suspending and reacting the fluorinatedvinyl compound and a gas of the fluorinated olefin in an aqueoussolution of a suspension stabilizer to thereby carry out thepolymerization.

In the present embodiment 1, any resin precursor fabricated by anypolymerization method described above can be used. Any block-shape ortaper-shape polymer obtained by regulating the polymerization conditionsuch as the amount of TFE gas supplied may be used as the resinprecursor.

The resin precursor may be one prepared by treating impure terminals andstructurally easily-oxidizable moieties (CO group-, H-bonded moietiesand the like) produced in a resin molecular structure during thepolymerization reaction by a well-known method under fluorine gas tothereby fluorinate the moieties.

In the resin precursor, a part of ion-exchange group precursor groups(for example, SO₂F groups) may be partially (including intermolecularly)imidized (e.g., alkylimidized).

The molecular weight of the resin precursor is not especially limited,but in terms of a value of a melt flow index (MFI) of the precursormeasured according to ASTM: D1238 (measurement conditions: a temperatureof 270° C. and a load of 2,160 g), is preferably 0.05 to 50 (g/10 min),more preferably 0.1 to 30 (g/10 min), and still more preferably 0.5 to20 (g/10 min).

The shape of the resin precursor is not especially limited, but from theviewpoint of accelerating treatment rates in a hydrolysis treatment andan acid treatment described later, is preferably a pellet-shape of 0.5cm³ or smaller, a disperse liquid or a powdery particle-shape; and amongthese, powdery bodies after the polymerization are preferably used. Fromthe viewpoint of the costs, an extruded film-shape resin precursor maybe used.

A method for producing a fluorine-based polyelectrolyte polymer of thepresent embodiment 1 from the resin precursor is not especially limited,and examples thereof include a method in which the resin precursor isextruded through a nozzle, a die, or the like by using an extruder, andthereafter is subjected to a hydrolysis treatment, and a method in whichthe resin precursor product as it is on the polymerization, that is, adisperse-liquid product, or a product made powdery by precipitation andfiltration is thereafter subjected to a hydrolysis treatment.

More specifically, a resin precursor obtained as in the above, and asrequired, molded is then immersed in a basic reaction liquid to bethereby subjected to a hydrolysis treatment. The basic reaction liquidused in the hydrolysis treatment is not especially limited, butpreferable are an aqueous solution of an amine compound such asdimethylamine, diethyleamine, monomethylamine, or monoethylamine, and anaqueous solution of a hydroxide of an alkali metal or an alkaline earthmetal; and especially preferable are aqueous solutions of sodiumhydroxide and potassium hydroxide. In the case of using a hydroxide ofan alkali metal or an alkaline earth metal, the content thereof is notespecially limited, but preferably 10 to 30% by mass with respect to thewhole of a reaction liquid. The reaction liquid more preferably furthercontains a swelling organic compound such as methyl alcohol, ethylalcohol, acetone, and dimethyl sulfoxide (DMSO). The content of aswelling organic compound is preferably 1 to 30% by mass with respect tothe whole of the reaction liquid.

The resin precursor is subjected to a hydrolysis treatment in the basicreaction liquid, thereafter sufficiently washed with warm water or thelike, and thereafter subjected to an acid treatment. An acid used in theacid treatment is not especially limited, but is preferably a mineralacid such as hydrochloric acid, sulfuric acid, or nitric acid, or anorganic acid such as oxalic acid, acetic acid, formic acid, ortrifluoroacetic acid, and more preferably a mixture of these acids andwater. The above acids may be used singly or in combinations of two ormore. A basic reaction liquid used in the hydrolysis treatment may beremoved by a treatment with a cation-exchange resin or the likepreviously before the acid treatment.

An ion-exchange group precursor group of a resin precursor is protonatedby an acid treatment to thereby produce an ion-exchange group. Forexample, in the case of a PFSA resin precursor produced using afluorinated vinyl ether compound represented by the above formula (5), Win the formula (5) is protonated by an acid treatment to thereby makeSO₃H. A fluorine-based polyelectrolyte polymer obtained by thehydrolysis treatment and acid treatment is enabled to be dispersed ordissolved in a protonic organic solvent, water, or a mixed solvent ofthe both.

In the fluorine-based polyelectrolyte polymer in the present embodiment1, a part of the molecular chain terminals is fluorinated. Since thedecomposition of the fluorine-based polyelectrolyte polymer issuppressed during the operation of a redox flow secondary battery byfluorinating and thereby stabilizing unstable functional groups presentat a part of molecular chain terminals of the polymer, an electrolytemembrane for a redox flow secondary battery excellent in the durabilitycan be provided.

A method of fluorinating molecular chain terminals of a fluorine-basedpolyelectrolyte polymer is not especially limited, but for example, mayinvolve treating impure terminals and structurally easily-oxidizablemoieties (CO group-, H-bonded moieties and the like) produced in a resinmolecular structure during the above-mentioned polymerization reactionof the fluorine-based polyelectrolyte polymer by a well-known methodunder fluorine gas to thereby fluorinate the moieties. Specifically, apart of molecular chain terminals can be fluorinated, for example, bysubjecting a perfluorocarbonsulfonic acid resin having a precursor groupof a sulfonic acid group to a heat treatment at a temperature of 200 to300° C. under reduced pressure of a pressure of 0.02 MPa or lower for0.1 hours or longer, and thereafter bringing the heat-treated resin intocontact with fluorine gas at a temperature of 150 to 200° C.

Here, the fluorination of molecular chain terminals of a fluorine-basedpolyelectrolyte polymer can be evaluated by the following Fenton'sreagent immersion test.

In a test in which 50 g of a Fenton's reagent solution containing a 3%hydrogen peroxide solution and 200 ppm of divalent iron ions isprepared; thereafter, within 1 min, 0.1 g of the polymer is immersed inthe Fenton's reagent solution at 40° C. for 16 hours under no stirring,and the fluorination is evaluated by a variation in the amount of thefluorine ions eluted detected in the solution before and after the test.Here, the amount of the fluorine ions eluted detected in the solution is0.03% or smaller of the whole amount of fluorine in the polymerimmersed, preferably 0.01% or smaller, and more preferably 0.002% orsmaller. If the amount of the fluorine ions eluted is 0.03% or smallerof the whole amount of fluorine in the polymer immersed, the amount ofunstable terminal groups is small, and the decrease in the voltagehardly occurs, in the battery operation for a long time.

The shape of a sample for the Fenton's reagent immersion test may be apolyelectrolyte polymer, or may be a film prepared by forming as a filma dispersion liquid containing the polymer by a well-known method.

The amount of the fluorine ions eluted makes an index of the resistanceto the decomposition of the polymer in the operation of a redox flowsecondary battery.

(Ion-Exchange Resin Composition)

The content of a fluorine-based polyelectrolyte polymer having astructure represented by the above formula (1) contained in anion-exchange resin composition forming an electrolyte membrane in thepresent embodiment 1 is not especially limited, but the ion-exchangeresin composition preferably contains as a main component thefluorine-based polyelectrolyte polymer having the above specificstructure from the viewpoint of the ion permselectivity and theoxidative deterioration resistance. Here, “containing as a maincomponent” refers to a lower limit value of the content in the resincomposition of about 33.3% by mass, preferably 40% by mass, morepreferably 50% by mass, still more preferably 50% by mass, further stillmore preferably 80% by mass, and especially preferably 90% by mass. Theupper limit value is not especially limited, but is preferably 99.5% bymass or less.

An ion-exchange resin composition in the present embodiment 1 maycontain fluorine-based resins (fluorine-based resins containingcarboxylic acid, phosphoric acid or the like, and other well-knownfluorine-based resins) other than a fluorine-based polyelectrolytepolymer represented by the formula (1). The fluorine-based resin iscontained, with respect to 100 parts by mass of a fluorine-basedpolyelectrolyte polymer represented by the formula (1) used in thepresent embodiment, preferably in 30 to 50 parts by mass, morepreferably in 10 to 30 parts by mass, and still more preferably 0 to 10parts by mass.

In the case of using two or more of these resins, a mixing method is notespecially limited, and may involve dissolving in a solvent ordispersing in a medium and mixing the resins, or may involveextrusion-mixing resin precursors.

The fluorine-based polyelectrolyte polymer may be contained singly in aform of a partial salt (about 0.01 to 5 equivalent % of the equivalentof the whole ion-exchange group) with an alkali metal, an alkaline earthmetal, or besides, a radical-decomposable transition metal (Ce-basedadditive, Co-based additive, Mn-based additive, or the like), or in aform in concurrent use therewith of a basic polymer described later.

If an ion-exchange resin composition in the present embodiment 1contains, in addition to the above-mentioned fluorine-basedpolyelectrolyte polymer, a basic polymer (including a low molecularweight substance such as an oligomer), the chemical stability (mainlythe oxidation resistance and the like) as the resin composition islikely to increase, which is therefore preferable. These compoundspartially make ion complexes in a microparticulate form or a form nearmolecular dispersion in the resin composition, and form an ionicallycrosslinked structure. Particularly in the case where EW of afluorine-based polyelectrolyte polymer is low (for example, in the caseof 300 to 500), these compounds are preferable from the viewpoint of thebalance among the water resistance, the electric resistance, and thelike.

(Equilibrium Moisture Content)

The equilibrium moisture content of an electrolyte membrane in thepresent embodiment 1 is preferably 5% by mass or higher, more preferably10% by mass or higher, and still more preferably 15% by mass or higher.The equilibrium moisture content of an electrolyte membrane in thepresent embodiment 1 is preferably 80% by mass or lower, more preferably50% by mass or lower, and still more preferably 40% by mass or lower. Ifthe equilibrium moisture content of an electrolyte membrane is 5% bymass or higher, the electric resistance, the current efficiency, theoxidation resistance, and the ion permselectivity of the membrane arelikely to be good. By contrast, if the equilibrium moisture content is80% by mass or lower, the dimensional stability and the strength of themembrane are likely to be good and the increase of water-solublecomponents is likely to be suppressed. The equilibrium moisture contentof an electrolyte membrane is expressed as an equilibrium (being leftfor 24 hours) saturated water absorption rate (Wc) at 23° C. and 50%relative humidity (RH), based on the membrane prepared by forming amembrane from a dispersion liquid of the resin composition with waterand an alcoholic solvent, and drying the membrane at 160° C. or lower.

The equilibrium moisture content of an electrolyte membrane can beregulated by the similar method as in EW described above.

(Method for Producing an Electrolyte Membrane)

A production method of an electrolyte membrane (membrane formationmethod) in the present embodiment 1 is not especially limited, and awell-known extrusion method or cast membrane formation method can beused. The electrolyte membrane may be of a single layer or of amultilayer (2 to 5 layers); and in the case of a multilayer, theperformance of the electrolyte membrane can be improved by laminatingmembranes having different properties (for example, resins havingdifferent EWs and functional groups). In the case of a multilayer, thelamination may be carried out at the extrusion membrane production timeor the cast time, or each membrane obtained may be laminated.

The electrolyte membrane formed in the above method is sufficientlywashed with water (or, as required, before water washing, treated withan aqueous acidic liquid such as dilute hydrochloric acid, nitric acid,or sulfuric acid) to thereby remove impurities, and is preferablysubjected to a heat treatment in the air or an inert gas (preferably inan inert gas) preferably at 130 to 200° C., more preferably at 140 to180° C., and still more preferably 150 to 170° C., for 1 to 60 min. Thetime of the heat treatment is more preferably 1 to 30 min, still morepreferably 2 to 20 min, further still more preferably 3 to 15 min, andespecially preferably about 5 to 10 min.

The resin in the state as it is at the membrane formation time isusually not sufficiently entangled among particles (among primaryparticles and secondary particles) and molecules originated from rawmaterials. One reason for carrying out the above heat treatment is thepurpose of interparticulately and intermolecularly entangling the resin,and is particularly in stabilizing the water resistance (particularlydecreasing the hot water-dissolving component ratio) and the saturatedwater absorption rate of water, and producing stable clusters. The heattreatment is useful also from the viewpoint of the improvement of themembrane strength. Particularly in the case of using the cast membraneformation method, the heat treatment is useful.

Another reason for carrying out the above heat treatment is because theformation of fine intermolecular crosslinking among molecules of afluorine-based polyelectrolyte polymer presumably contributes to theformation of clusters excellent in the water resistance and stable, andprovides an effect of making the cluster diameter uniform and small.

A further reason is because the above heat treatment presumably causesat least a part of ion-exchange groups of a fluorine-basedpolyelectrolyte polymer in an ion-exchange resin composition to reactwith active reaction sites (aromatic rings and the like) of otheradditive (including resins) components to thereby form fine crosslinkingthrough the reaction (particularly the reaction of ion-exchange groupspresent near the other resin components being dispersed additives) andcontribute to the stabilization. The degree of the crosslinking is, interms of EW (the degree of the EW decrease before and after the heattreatment), preferably 0.001 to 5%, more preferably 0.1 to 3%, and stillmore preferably about 0.2 to 2%.

Carrying out the above heat treatment under the above suitable condition(time, temperature) is preferable from the viewpoint of exhibiting theeffect of the heat treatment, and from the viewpoint of suppressing thedegradation of the oxidative deterioration resistance during actualusage as an electrolyte membrane, with the degradation starting fromfaults generated in the molecular structure due to the generation andincrease of fluorine removal, hydrofluoric acid removal, sulfonic acidremoval and thermally oxidized sites, and the like.

The Present Embodiment 2

A redox flow secondary battery in the present embodiment 2, comprisingan electrolytic bath comprising:

a positive electrode cell chamber comprising a positive electrodecomposed of a carbon electrode;

a negative electrode cell chamber comprising a negative electrodecomposed of a carbon electrode; and

an electrolyte membrane as a separation membrane to separate thepositive electrode cell chamber and the negative electrode cell chamber,

wherein the positive electrode cell chamber comprises a positiveelectrode electrolyte solution comprising a positive electrode activesubstance; and the negative electrode cell chamber comprises a negativeelectrode electrolyte solution comprising a negative electrode activesubstance;

wherein the redox flow secondary battery charges and discharges based onchanges in valences of the positive electrode active substance and thenegative electrode active substance in the electrolyte solutions;

wherein the electrolyte membrane comprises an ion-exchange resincomposition comprising a fluorine-based polyelectrolyte polymer having astructure represented by the following formula (1):

—[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1)

wherein X¹, X², and X³ each independently represent one or more selectedfrom the group consisting of halogen atoms and perfluoroalkyl groupshaving 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZwherein Z represents a hydrogen atom, an alkali metal atom, an alkalineearth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄)wherein R₁, R₂, R₃, and R₄ each independently represent one or moreselected from the group consisting of alkyl groups and arene groups,when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² eachindependently represent one or more selected from the group consistingof halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groupshaving 1 to 10 carbon atoms; and a and g represent numbers satisfying0≦a<1, 0<g≦1, and a+g=1, b represents an integer of 0 to 8, c represents0 or 1, and d, e, and f each independently represent an integer of 0 to6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein the ion-exchange resin composition comprises a Ce-basedadditive.

An ion-exchange resin composition forming an electrolyte membrane in thepresent embodiment 2 contains a Ce-based additive. The incorporation ofthe Ce-based additive in the ion-exchange resin composition conceivablyion-exchanges cerium ions for a part of ion-exchange groups contained inthe electrolyte membrane, resulting in improving the ion permselectivityand the oxidative deterioration resistance.

The Ce-based additive is preferably one to form cerium ions having avalence of +3 and/or +4 in a solution. Examples of salts containingcerium ions having a valence of +3 include cerium nitrate, ceriumcarbonate, cerium acetate, cerium chloride, and cerium sulfate. Examplesof salts containing cerium ions having a valence of +4 include ceriumsulfate (Ce(SO₄)₂.4H₂O), cerium diammonium nitrate (Ce(NH₄)₂(NO₃)₆), andcerium tetraammonium sulfate (Ce(NH₄)₄(SO₄)₄.4H₂O). As a Ce-basedadditive, an organometal complex salt of cerium can also be used, and anexample thereof includes cerium acetylacetonate(Ce(CH₃COCHCOCH₃)₃.3H₂O). Among the above, cerium nitrate and ceriumsulfate are especially preferable because of being water-soluble andbeing likely to be easily handled.

The content of a Ce-based additive is, in a proportion of cerium ionswith respect to the number of ion-exchange groups in an electrolytemembrane, preferably 0.02 to 20%, more preferably 0.05 to 15%, and stillmore preferably 0.07 to 10%. If the content of a Ce-based additive is20% or lower, the ion permselectivity is likely to be good; and if 0.02%or higher, the oxidative deterioration resistance (hydroxy radicalresistance) is likely to be improved.

If an ion-exchange resin composition in the present embodiment 2contains, in addition to the above-mentioned fluorine-basedpolyelectrolyte polymer and the Ce-based additive, an alkali metal, analkaline earth metal, a radical-decomposable transition metal (Co-basedadditive, Mn-based additive, or the like), and a basic polymer(including a low molecular weight substance such as an oligomer), thechemical stability (mainly the oxidation resistance and the like) as theresin composition is likely to increase. These compounds partially makeion complexes in a microparticulate form or a form near moleculardispersion in the resin composition, and form an ionically crosslinkedstructure. Particularly in the case where EW of a fluorine-basedpolyelectrolyte polymer is low (in the case of 300 to 500), thesecompounds are preferable from the viewpoint of the balance among thewater resistance, the electric resistance, and the like.

In the present embodiment 2, since each member constituting a redox flowsecondary battery and an electrolyte membrane, and physical propertiesthereof are similar to those in the present embodiment 1, thedescription thereof will be omitted.

The Present Embodiment 3

A redox flow secondary battery in the present embodiment 3, comprisingan electrolytic bath comprising:

a positive electrode cell chamber comprising a positive electrodecomposed of a carbon electrode;

a negative electrode cell chamber comprising a negative electrodecomposed of a carbon electrode; and

an electrolyte membrane as a separation membrane to separate thepositive electrode cell chamber and the negative electrode cell chamber,

wherein the positive electrode cell chamber comprises a positiveelectrode electrolyte solution comprising a positive electrode activesubstance; and the negative electrode cell chamber comprises a negativeelectrode electrolyte solution comprising a negative electrode activesubstance;

wherein the redox flow secondary battery charges and discharges based onchanges in valences of the positive electrode active substance and thenegative electrode active substance in the electrolyte solutions;

wherein the electrolyte membrane comprises an ion-exchange resincomposition comprising a fluorine-based polyelectrolyte polymer having astructure represented by the following formula (1):

—[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1)

wherein X¹, X², and X³ each independently represent one or more selectedfrom the group consisting of halogen atoms and perfluoroalkyl groupshaving 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZwherein Z represents a hydrogen atom, an alkali metal atom, an alkalineearth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄)wherein R₁, R₂, R₃, and R₄ each independently represent one or moreselected from the group consisting of alkyl groups and arene groups,when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² eachindependently represent one or more selected from the group consistingof halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groupshaving 1 to 10 carbon atoms; and a and g represent numbers satisfying0≦a<1, 0<g≦1, and a+g=1, b represents an integer of 0 to 8, c represents0 or 1, and d, e, and f each independently represent an integer of 0 to6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein the ion-exchange resin composition comprises a Co-based and/or aMn-based additive.

An ion-exchange resin composition forming an electrolyte membrane in thepresent embodiment 3 contains a Co-based and/or a Mn-based additive. Theincorporation of the Co-based and/or the Mn-based additive in theion-exchange resin composition conceivably ion-exchanges cobalt ionsand/or manganese ions for a part of ion-exchange groups contained in theelectrolyte membrane, resulting in improving the ion permselectivity andthe oxidative deterioration resistance.

The Co-based additive is preferably one to form cobalt ions having avalence of +2 and/or +3 in a solution. Examples of salts containingcobalt ions having a valence of +2 include cobalt nitrate, cobaltcarbonate, cobalt acetate, cobalt chloride, and cobalt sulfate. Examplesof salts containing cobalt ions having a valence of +3 include cobaltchloride (CoCl₃) and cobalt nitrate (Co(NO₃)₂). As a Co-based additive,an organometal complex salt of cobalt can also be used, and an examplethereof includes cobalt acetylacetonate (Co(CH₃COCHCOCH₃)₃). Among theabove, cobalt nitrate and cobalt sulfate are preferable because of beingwater-soluble and being likely to be easily handled.

As the Mn-based additive, various types of compounds can be used such aswater-soluble manganese salts, water-insoluble manganese salts, andinsoluble compounds including oxides and hydroxides. The valence ofmanganese is +2 or +3. Examples of salts containing manganese ionshaving a valence of +2 include manganese acetate (Mn(CH₃COO)₂.4H₂O),manganese chloride (MnCl₂.4H₂O), manganese nitrate (Mn(NO₃)₂.6H₂O),manganese sulfate (MnSO₄.5H₂O), and manganese carbonate (MnCO₃.nH₂O).Examples of salts containing manganese ions having a valence of +3include manganese acetate (Mn(CH₃COO)₃.2H₂O). Organometal complex saltsof manganese can also be used, and an example thereof includes manganeseacetylacetonate (Mn(CH₃COCHCOCH₃)₂). Among the above, manganese nitrateand manganese sulfate are preferable because of being water-soluble andbeing likely to be easily handled.

The content of a Co-based and/or a Mn-based additive is, in a proportionof cobalt ions and/or manganese ions with respect to the number ofion-exchange groups in an electrolyte membrane, preferably 0.01 to 50%,more preferably 0.05 to 30%, and still more preferably 0.07 to 20%. Ifthe content of a Co-based and/or a Mn-based additive is 50% or lower,the ion permselectivity is likely to be good; and if 0.01% or higher,the oxidative deterioration resistance (hydroxy radical resistance) islikely to be improved.

If an ion-exchange resin composition in the present embodiment contains,in addition to the above-mentioned fluorine-based polyelectrolytepolymer and the Co-based and/or Mn-based additive, an alkali metal, analkaline earth metal, a radical-decomposable transition metal (Ce-basedadditive and the like), and a basic polymer (including a low molecularweight substance such as an oligomer), the chemical stability (mainlythe oxidation resistance and the like) as the resin composition islikely to increase. These compounds partially make ion complexes in amicroparticulate form or a form near molecular dispersion in the resincomposition, and form an ionically crosslinked structure. Particularlyin the case where EW of a fluorine-based polyelectrolyte polymer is low(in the case of 300 to 500), these compounds are preferable from theviewpoint of the balance among the water resistance, the electricresistance, and the like.

In the present embodiment 3, since each member constituting a redox flowsecondary battery and an electrolyte membrane, and physical propertiesthereof are similar to those in the present embodiment 1, thedescription thereof will be omitted.

The Present Embodiment 4

A redox flow secondary battery in the present embodiment 4, comprisingan electrolytic bath comprising:

a positive electrode cell chamber comprising a positive electrodecomposed of a carbon electrode;

a negative electrode cell chamber comprising a negative electrodecomposed of a carbon electrode; and

an electrolyte membrane as a separation membrane to separate thepositive electrode cell chamber and the negative electrode cell chamber,

wherein the positive electrode cell chamber comprises a positiveelectrode electrolyte solution comprising a positive electrode activesubstance; and the negative electrode cell chamber comprises a negativeelectrode electrolyte solution comprising a negative electrode activesubstance;

wherein the redox flow secondary battery charges and discharges based onchanges in valences of the positive electrode active substance and thenegative electrode active substance in the electrolyte solutions;

wherein the electrolyte membrane comprises an ion-exchange resincomposition comprising a fluorine-based polyelectrolyte polymer having astructure represented by the following formula (1):

—[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1)

wherein X¹, X², and X³ each independently represent one or more selectedfrom the group consisting of halogen atoms and perfluoroalkyl groupshaving 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZwherein Z represents a hydrogen atom, an alkali metal atom, an alkalineearth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄)wherein R₁, R₂, R₃, and R₄ each independently represent one or moreselected from the group consisting of alkyl groups and arene groups,when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² eachindependently represent one or more selected from the group consistingof halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groupshaving 1 to 10 carbon atoms; and a and g represent numbers satisfying0≦a<1, 0<g≦1, and a+g=1, b represents an integer of 0 to 8, c represents0 or 1, and d, e, and f each independently represent an integer of 0 to6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein the ion-exchange resin composition comprises 0.1 to 20 parts bymass of a polyphenylene ether resin and/or a polyphenylene sulfide resinwith respect to 100 parts by mass of the fluorine-based polyelectrolytepolymer.

An ion-exchange resin composition in the present embodiment 4, from theviewpoint of the oxidation resistance and the cluster diameter of anelectrolyte membrane, contains a polyphenylene ether resin (hereinafter,also referred to as “PPE resin”) and/or a polyphenylene sulfide resin(hereinafter, also referred to as “PPS resin”).

PPE and PPS resins can be added by a method of mixing these to a resincomposition containing a fluorine-based polyelectrolyte polymer by anextrusion method, or a method of mixing an aqueous solvent dispersion ofPPE and PPS resins to a stock dispersion of a resin compositioncontaining a fluorine-based polyelectrolyte polymer.

The amount of PPE and PPS resins added, with respect to 100 parts bymass of a fluorine-based polyelectrolyte polymer having a structurerepresented by the above formula (1), is 0.1 to 20 parts by mass, andpreferably 0.5 to 10 parts by mass. In the case where the content of aPPE resin and/or a PPS resin is 0.1 parts by mass or higher, theoxidation resistance and the ion permselectivity of an electrolytemembrane are improved; and in the case of 20 parts by mass or lower, asufficient membrane strength can be provided.

The PPS resin in the present embodiment is preferably a PPS resincontaining 70 mol % or more of a paraphenylene sulfide skeleton, andpreferably 90 mol % or more thereof. A method for producing a PPS resinis not especially limited, and includes a method in which usually ahalogen-substituted aromatic compound, for example, p-dichlorobenzene ispolymerized in the presence of sulfur and sodium carbonate, a method ofthe polymerization in the presence of sodium sulfide or sodiumhydrosulfide and sodium hydroxide in a polar solvent, a method of thepolymerization in the presence of hydrogen sulfide and sodium hydroxideor sodium aminoalkanoate in a polar solvent, and a method ofself-condensation of p-chlorothiophenol; but among these, a method issuitable in which sodium sulfide and p-dichlorobenzene are reacted in anamide-based solvent such as N-methylpyrrolidone or dimethylacetamide, ora sulfone-based solvent such as sulfolane. Specifically, there can beused methods described, for example, in U.S. Pat. No. 2,513,188,Japanese Patent Publication Nos. 44-27671, 45-3368, and 52-12240,Japanese Patent Laid-Open No. 61-225217, U.S. Pat. No. 3,274,165,British Patent No. 1160660, Japanese Patent Publication No. 46-27255,Belgian Patent No. 29437, and Japanese Patent Laid-Open No. 5-222196,and there can be used methods of prior arts exemplified in theseofficial gazettes.

The melt viscosity (a value acquired by holding at 300° C. and a load of196 N for 6 min using a flow tester whose L/D (L: orifice length, D:orifice diameter)=10/1) at 320° C. of a PPS resin is preferably 1 to10,000 poises, and more preferably 100 to 10,000 poises.

A PPS resin into which an acidic functional group has been incorporatedcan further be used suitably. As an acidic functional group to beincorporated, preferable are a sulfonic acid group, a phosphoric acidgroup, a carboxylic acid group, a maleic acid group, a maleic anhydridegroup, a fumaric acid group, an itaconic acid group, an acrylic acidgroup, and a methacrylic acid group; and especially preferable is asulfonic acid group.

A method of incorporating an acidic functional group is not especiallylimited, and usual methods can be used. The incorporation of a sulfonicgroup can be carried out, for example, by using a sulfonating agent suchas sulfuric anhydride or fuming sulfuric acid under the well-knowncondition; specifically, the incorporation can be carried out under theconditions described in K. Hu, T. Xu, W. Yang, Y. Fu, Journal of AppliedPolymer Science, Vol. 91, and E. Montoneri, Journal of Polymer Science:Part A: Polymer Chemistry, Vol. 27, 3043-3051 (1989).

Also a PPS resin can suitably be used which is prepared by substitutingthe acidic functional group incorporated in a PPS resin with a metalsalt or an amine salt. In this case, as the metal salt, preferably usedare alkali metal salts such as sodium salts and potassium salts, andalkaline earth metal salts such as calcium salts.

The PPE resin is not especially limited, and examples thereof includepoly(2,6-dimethyl-1,4-phenylene ether),poly(2-methyl-6-ethyl-1,4-phenylene ether),poly(2-methyl-6-phenyl-1,4-phenylene ether), andpoly(2,6-dichloro-1,4-phenylene ether), and also include polyphenyleneether copolymers such as copolymers of 2,6-dimethylphenol with otherphenols (for example, 2,3,6-trimethylphenol and 2-methyl-6-butylphenol).Among these, poly(2,6-dimethyl-1,4-phenylene ether) and a copolymer of2,6-dimethylphenol with 2,3,6-trimethylphenol are preferable, andpoly(2,6-dimethyl-1,4-phenylene ether) is especially preferable.

A method for producing a PPE resin is not especially limited; and a PPEresin can easily be produced, for example, by oxidatively polymerizing2,6-xylenol with the use of a complex of a cuprous copper salt with anamine as a catalyst, as described in U.S. Pat. No. 3,306,874. PPE resinscan easily be produced also by methods described in U.S. Pat. Nos.3,306,875, 3,257,357, and 3,257,358, Japanese Patent Publication No.52-17880, Japanese Patent Laid-Open Nos. 50-51197 and 63-152628, and thelike.

In addition to a single PPE described above, also a PPE resin cansuitably be used which is prepared by blending a polystyrene (includingan atactic high-impact polystyrene) having atactic or syndiotacticstereoregularity in the range of 1 to 400 parts by mass with respect to100 parts by mass of the PPE component described above.

Also PPE resins can suitably be used which are prepared by incorporatingreactive functional groups to various types of PPE described above. Thereactive functional groups include an epoxy group, an oxazonyl group, anamino group, an isocyanate group, a carbodiimide group, and other acidicfunctional groups; and among these, acidic functional groups are moresuitably used. An acidic functional group to be incorporated is notespecially limited, but preferable are a sulfonic acid group, aphosphoric acid group, a carboxylic acid group, a maleic acid group, amaleic anhydride group, a fumaric acid group, an itaconic acid group, anacrylic acid group, and a methacrylic acid group; and especiallypreferable is a sulfonic acid group.

The weight-average molecular weight of a PPE resin is preferably 1,000or higher and 5,000,000 or lower, and more preferably 1,500 or higherand 1,000,000 or lower. Here, the weight-average molecular weight refersto a value measured by gel permeation chromatography (GPC).

In the present embodiment 4, since each member constituting a redox flowsecondary battery and an electrolyte membrane, and physical propertiesthereof are similar to those in the present embodiment 1, thedescription thereof will be omitted.

The electrolyte membranes in the present embodiments 1 to 4 areexcellent in the ion permselectivity, low in the electric resistance,and excellent also in the durability (mainly the hydroxy radicaloxidation resistance), and exhibit excellent performance as a separationmembrane for a redox flow secondary battery. Here, each physicalproperty in the present specification can be measured according tomethods described in the following Examples unless otherwise specified.

EXAMPLES

Then, the present embodiments will be described more specifically by wayof Examples and Comparative Examples, but the present embodiments arenot limited to the following Examples unless going over their gist.

[Measurement Methods] (1) The Melt Flow Index of a PFSA Resin Precursor

The melt flow index was measured according to ASTM: D1238 under themeasurement conditions of a temperature of 270° C. and a load of 2,160g.

(2) The Measurement of an Equivalent Weight EW of a PFSA Resin

0.3 g of a PFSA resin was immersed in 30 mL of a saturated NaCl aqueoussolution at 25° C., and left for 30 min under stirring. Then, freeprotons in the saturated NaCl aqueous solution was subjected to aneutralization titration using a 0.01 N sodium hydroxide aqueoussolution with phenolphthalein as an indicator. The end point of theneutralization titration was set at a pH of 7; and the PFSA resincontent, obtained after the neutralization titration, in which counterions of ion-exchange groups were in the sodium ion state was rinsed withpure water, further dried in a pan drier at 160° C., and weighed. Theamount of substance of sodium hydroxide used for the neutralization wastaken as M (mmol), and the mass of the PFSA resin in which counter ionsof the ion-exchange groups were in the sodium ion state was taken as W(mg); and the equivalent weight EW (g/eq) was determined from thefollowing expression.

EW=(W/M)−22

The above operation was repeated five times; and the maximum value andthe minimum value of the five calculated EW values were excluded, andthe three values were arithmetically averaged to thereby make ameasurement result.

(3) Membrane Thickness

An electrolyte membrane was allowed to stand still in a 23° C., 50% RHconstant-temperature constant-humidity chamber for 1 hour or longer, andthereafter, the membrane thickness was measured using a film thicknessmeter (made by Toyo Seki Seisaku-sho, Ltd., trade name: “B-1”).

(4) the Measurements of an Equilibrium Moisture Content and a MaximumMoisture Content

A dispersion liquid of a PFSA resin was coated on a clear glass plate,dried at 150° C. for about 10 min, and peeled to thereby form a membraneof about 30 μm; the membrane was left in water at 23° C. for about 3hours, and thereafter left in a room of a relative humidity (RH) of 50%for 24 hours; and then, the equilibrium moisture content was measured.An 80° C.-vacuum-dried membrane was used as the reference driedmembrane. The equilibrium moisture content was calculated from the massvariation in the membrane. A maximum value observed in the equilibriummoisture content measurement was taken as the maximum moisture content.

(5) the Measurement of an Amount of Fluorine Ions Eluted

A PFSA resin was held in a glove box in which nitrogen was made to flowfor 24 hours; about 0.1 g of the PFSA resin was weighed in the glovebox; within 1 min after 50 g of a Fenton's reagent solution containing a3% hydrogen peroxide solution and 200 ppm of divalent iron ions wasprepared, the weighed PFSA resin was immersed in the Fenton's reagentsolution at 40° C. for 16 hours under no stirring. A solution mass wasmeasured after the membrane was removed; and the fluorine ionconcentration in the solution was measured by an ion meter, and theamount of the fluorine ions eluted was calculated.

(6) Charge and Discharge Test

In a redox flow secondary battery, each of liquid-permeable porouscurrent collector electrodes (for a negative electrode and for apositive electrode) was disposed on either side of the separationmembrane, and these were held by pressing; one side partitioned by theseparation membrane was made a positive electrode cell chamber and theother side was made a negative electrode cell chamber; and thethicknesses of both the cell chambers were secured by spacers. Chargeand discharge of the battery was carried out by circulating a positiveelectrode electrolyte solution composed of a sulfuric acid electrolytesolution comprising tetravalent vanadium (V⁴⁺) and pentavalent vanadium(V⁵⁺) to the positive electrode cell chamber, and circulating a negativeelectrode electrolyte solution comprising trivalent vanadium (V³⁺) anddivalent vanadium (V²⁺) to the negative electrode cell chamber. In thecharge time therein, in the positive electrode cell chamber, vanadiumions released electrons to thereby oxidize V⁴⁺ to V⁵⁺; and in thenegative electrode cell chamber, electrons having returned through anexternal circuit reduced V³⁺ to V²⁺. In the oxidation and reductionreactions, in the positive electrode cell chamber, protons (H⁺) becameexcessive; by contrast, in the negative electrode cell chamber, protons(H⁺) became insufficient. The excessive protons in the positiveelectrode cell chamber selectively migrated to the negative electrodechamber through the separation membrane to thereby hold the electricneutrality. In the discharge time, a reaction reverse theretoprogressed. The battery efficiency (energy efficiency) (%) at this timeis represented by a ratio (%) obtained by dividing a discharge electricenergy by a charge electric energy;

and both the electric energies depend on the internal resistance of thebattery cells, the ion permselectivity of the separation membrane, andthe current losses of others. The current efficiency (%) is representedby a ratio (%) obtained by dividing an amount of discharge electricityby an amount of charge electricity; and both the amounts of electricitydepend on the ion permselectivity of the separation membrane and currentlosses of others. The battery efficiency is represented by a product ofthe current efficiency and a voltage efficiency. Since the reduction ofthe internal resistance, that is, cell electric resistivity, improvesthe battery efficiency (energy efficiency) and the improvement of theion permselectivity and the reduction of the current losses of othersimprove the current efficiency, these factors become important indicesin the redox flow secondary battery.

A charge and discharge test was carried out using a battery thusobtained. An aqueous electrolyte solution having a whole vanadiumconcentration of 2 M/L and a whole sulfate ion concentration of 4 M/Lwas used; the thicknesses of the positive electrode cell chamber and thenegative electrode cell chamber installed were each 5 mm; and a porousfelt of 5 mm in thickness and about 0.1 g/cm³ in bulk density composedof a carbon fiber was interposed between the separation membrane andeach of both the porous electrodes. The charge and discharge test wascarried out at a current density of 80 mA/cm².

The cell electric resistivity (Ω·cm²) was determined by using the ACimpedance method, and measuring a direct-current resistance value at anAC voltage of 10 mV at a frequency of 20 kHz at the discharge initiationtime and multiplying the resistance value by the electrode area.

(7) Durability

The durability was evaluated using the current efficiency (%) and thecell electric resistivity (Ω·cm²) after 200 cycles of the charge anddischarge of the above (6) were carried out.

Example 1 (1) Production of a PFSA Resin Precursor

A 10% aqueous solution of C₇F₁₅COONH₄ and pure water were charged in astainless steel-made stirring-type autoclave, and the interioratmosphere of the autoclave was sufficiently replaced by vacuum andnitrogen; and thereafter, tetrafluoroethylene (CF₂═CF₂) (hereinafter,also referred to as “TFE”) gas was introduced, and the interior pressurewas pressurize to 0.7 MPa in terms of gage pressure. Then, an ammoniumpersulfuric acid aqueous solution was injected to initiate thepolymerization. While in order to supply TFE consumed by thepolymerization, TFE gas was continuously fed so as to hold the pressureof the autoclave at 0.7 MPa, CF₂═CFO(CF₂)₂—SO₂F of an amountcorresponding to 0.70 times the amount of TFE fed in mass ratio wascontinuously fed to carry out the polymerization by regulating thepolymerization condition in a best range to thereby obtain aperfluorocarbonsulfonic acid resin precursor powder. The MFI of theobtained PFSA resin precursor powder A1 was 1.5 (g/10 min).

(2) Fluorination Treatment of Molecular Chain Terminals

A multistage tray fabricated of a Hastelloy C alloy was placed in apressure-resistant reaction vessel of 50 L in interior volume whoseinner surface was fabricated of a Hastelloy C alloy; and a mixed gas of20% of fluorine gas and 80% of nitrogen gas was introduced at 0.25 MPain gage pressure, and the temperature was held at 190° C. for 4 hours tothereby subject the metal surface to a passivation treatment. Thetemperature was decreased, and thereafter, the above-mentioned PFSAresin precursor powder was put in the 50-L pressure-resistant vessel;and a mixed gas of 20% of fluorine gas and 80% of nitrogen gas wasintroduced at 0.25 MPa in gage pressure, and the temperature was held at180° C. for 4 hours to thereby subject the powder to a fluorinationtreatment. After the fluorination treatment, the fluorine gas wasexhausted; and a polymer was taken out, and crushed by a crusher tothereby obtain a fluorinated polymer (hereinafter, also referred to as“precursor polymer”) having —SO₂F groups being precursor groups ofsulfonic acid groups.

(3) Production of PFSA Acid Resins and Dispersion Solutions Thereof

The obtained precursor polymer was brought into contact with an aqueoussolution in which potassium hydroxide (15% by mass) and methyl alcohol(50% by mass) were dissolved at 80° C. for 20 hours to thereby subjectthe precursor polymer to a hydrolysis treatment. Thereafter, theprecursor polymer was immersed in water at 60° C. for 5 hours. Then,such a treatment that the resultant was immersed in a 2 N hydrochloricacid aqueous solution at 60° C. for 1 hour was repeated five times byrenewing the hydrochloric acid aqueous solution each time; andthereafter, the resultant was washed with ion-exchange water, and dried.A PFSA resin having a structure having sulfonic acid groups (SO₃H) andrepresented by the formula (2) (m=2) was thereby obtained. The EW of theobtained PFSA resin A1 was 650 (g/eq). The EW of a PFSA resin A2obtained using the PFSA resin precursor power A1 instead of theprecursor polymer was 650 (g/eq).

The obtained PFSA resins A1 and A2 were each put in a 5-L autoclavetogether with an ethanol aqueous solution (water:ethanol=50:50 (in massratio)), and the autoclave was hermetically closed; and the mixture washeated up to 160° C. under stirring by a blade, and the temperature washeld for 5 hours. Thereafter, the autoclave was spontaneously cooled,and a homogeneous dispersion liquid of 5% by mass of the PFSA resin wasthus fabricated. Then, 100 g of pure water was added to 100 g of thePFSA resin dispersion liquid, and stirred; and thereafter while thedispersion liquid was heated to 80° C. and stirred, the dispersionliquid was concentrated up to 20% by mass in terms of solid contentconcentration.

The PFSA resin dispersion liquids obtained from the PFSA resins A1 andA2 were named dispersion liquids (ASF1) and (ASF2), respectively.

(4) Production of an Electrolyte Membrane

The obtained dispersion liquid (ASF1) was cast on a polyimide film as acarrier sheet by a well-known usual method, exposed to hot air at 120°C. (for 20 min) to nearly completely evaporate the solvent and dry tothereby obtain a membrane. The membrane was further subjected to a heattreatment in a hot air atmosphere under the condition of 160° C. for 10min to thereby obtain an electrolyte membrane of 50 μm in membranethickness. The variation rate of EWs before and after the heat treatmentof the obtained electrolyte membrane was about 0.2 to 0.3%.

The equilibrium moisture content of the obtained electrolyte membranewas 12% by mass for ASF1.

The maximum moisture content of the electrolyte membrane in water at 25°C. for 3 hours was 23% by mass for ASF1.

Then, a charge and discharge test was carried out using the obtainedelectrolyte membrane as a separation membrane of a vanadium redox flowsecondary battery. The charge and discharge test was carried out byusing the membrane obtained from ASF1 and after the equilibrium wassufficiently reached in the electrolyte solution; and thereafter, afterthe stable state was made, the cell electric resistivity and the currentefficiency were measured; the current efficiency/the cell electricresistivity was (97.5/0.90) for the membrane of ASF1, thus giving anexcellent tendency.

Then, a durability test was carried out by using the membrane obtainedfrom ASF1 and carrying out 200 cycles of the charge and discharge andexamining the variation. As a result, the current efficiency (%)/thecell electric resistivity (Ω·cm²) was (97.3/0.90) for ASF1, giving onlya small variation and exhibiting excellent oxidation resistance.

The amount of the fluorine ions eluted was measured by using theobtained polymer, and was 0.001% of the whole amount of fluorine in theimmersed polymer.

Example 2

An electrolyte membrane was obtained by the similar method as in Example1, except for using Nafion DE2021CS (registered trademark, made by DuPont K. K., 20% solution) instead of the dispersion liquid (ASF1). Theequilibrium moisture content of the membrane was 6% by mass, and themaximum moisture content was 14% by mass.

As a result of carrying out a charge and discharge test by the similarmethod as in Example 1, the current efficiency (%)/the cell electricresistivity (Ω·cm²) was 94.5/1.20; and as a result of carrying out 200cycles of the charge and discharge as the durability test, the currentefficiency was 94%, and the electric resistance was 1.20.

The amount of the fluorine ions eluted was measured by using theobtained electrolyte membrane, and was 0.002% of the whole amount offluorine in the immersed polymer.

Example 3

An electrolyte membrane of 50 μm in membrane thickness obtained by thesimilar method as in Example 1 by using the dispersion liquid (ASF2) wasimmersed in a 1% cerium nitrate aqueous solution in which cerium nitratewas dissolved in distilled water, and stirred by a stirrer at roomtemperature for 40 hours to thereby incorporate cerium ions in theelectrolyte membrane. As a result of analyzing the cerium nitratesolution before and after the immersion by inductively coupled plasma(ICP) atomic emission spectrometry, the content of cerium ions in theelectrolyte membrane (a proportion of cerium ions with respect to thenumber of —SO₃— groups in the membrane) was 10.2%.

The equilibrium moisture content of the obtained electrolyte membranewas 12% by mass, and the maximum moisture content was 23% by mass.

As a result of carrying out a charge and discharge test by the similarmethod as in Example 1, the current efficiency (%)/the cell electricresistivity (Ω·cm²) was 97.5/0.90; and as a result of carrying out 200cycles of the charge and discharge as the durability test, the currentefficiency (%)/the cell electric resistivity (Ω·cm²) was 97.3/0.90,giving so small a variation, and exhibiting excellent oxidationresistance.

The amount of the fluorine ions eluted was measured by using theobtained electrolyte membrane, and was 0.008% of the whole amount offluorine in the immersed polymer.

Example 4

An electrolyte membrane was obtained by the similar method as in Example3, except for using Nafion DE2021 (registered trademark, made by Du PontK. K., 20% solution, EW: 1,050) instead of the dispersion liquid (ASF2)and using cerium carbonate instead of cerium nitrate. The equilibriummoisture content of the membrane was 6% by mass, and the maximummoisture content was 14% by mass.

As a result of carrying out a charge and discharge test by the similarmethod as in Example 1, the current efficiency (%)/the cell electricresistivity (Ω·cm²) was 94.5/1.20, in which the current efficiency wasin a lower level than in Example 3. This is presumably because theelectrolyte membrane of Example 4 had a slightly low ionpermselectivity. As a result of carrying out 200 cycles of the chargeand discharge as the durability test, the current efficiency was 94% andthe electric resistance was 1.20, also exhibiting slightly inferiordurability.

The amount of the fluorine ions eluted was measured by using theobtained electrolyte membrane, and was 0.01% of the whole amount offluorine in the immersed polymer.

Example 5 and 6

An electrolyte membrane of 50 μm in membrane thickness obtained by thesimilar method as in Example 1 by using the dispersion liquid (ASF2) wasimmersed in a 1% aqueous solution in which cobalt nitrate (Example 5) ormanganese acetate (Example 6) was dissolved in distilled water, andstirred by a stirrer at room temperature for 40 hours to therebyincorporate cobalt ions or manganese ions in the electrolyte membrane.As a result of analyzing the cobalt nitrate solution or the manganeseacetate solution before and after the immersion by inductively coupledplasma (ICP) atomic emission spectrometry, the content of cobalt ions ormanganese ions in the electrolyte membrane (a proportion of cobalt ionsor manganese ions with respect to the number of —SO₃— groups in thecorresponding membrane) was 14.7% or 13.8%, respectively.

The equilibrium moisture content of the obtained electrolyte membranewas 12% by mass for either membrane of Examples 5 and 6, and the maximummoisture content was 23% by mass for either.

As a result of carrying out a charge and discharge test by the similarmethod as in Example 1, the current efficiency (%)/the cell electricresistivity (Ω·cm²) was 97.5/0.90 for either; and as a result ofcarrying out 200 cycles of the charge and discharge as the durabilitytest, the current efficiency (%)/the cell electric resistivity (Ω·cm²)was 97.3/0.90, giving so small a variation, and exhibiting excellentoxidation resistance.

The amounts of the fluorine ions eluted were measured by using theobtained electrolyte membranes, and were 0.01% of the whole amounts offluorine in the immersed polymers.

Examples 7 and 8

Electrolyte membranes were obtained by the similar methods as inExamples 5 and 6, except for using Nafion DE2021 (made by Du Pont K. K.,20% solution, EW: 1,050) instead of the dispersion liquid (ASF2). Theequilibrium moisture contents of the membranes were 6% by mass foreither of the electrolyte membranes of Examples 7 and 8, and the maximummoisture contents were 14% by mass for either.

As a result of carrying out a charge and discharge test by the similarmethod as in Example 1, the current efficiency (%)/the cell electricresistivity (Ω·cm²) was 94.5/1.20 for either, in which the currentefficiencies were in a lower level than in Examples 5 and 6. This ispresumably because the electrolyte membranes of Examples 7 and 8 had aslightly low ion permselectivity. As a result of carrying out 200 cyclesof the charge and discharge as the durability test, the currentefficiency was 94% and the electric resistance was 1.20 for either, alsoexhibiting slightly inferior durability.

The amounts of the fluorine ions eluted were measured by using theobtained electrolyte membranes, and were 0.02% of the whole amounts offluorine in the immersed polymers.

Example 9

A PPS powder (made by Chevron Phillips Chemical Co. LP, type No. P-4)dissolved in an alkali aqueous solution (KOH-10% aqueous solution) washomogeneously mixed and dispersed under stirring in the PFSA resindispersion liquid (ASF2) so that the PPS powder was finally (in terms ofsolid component) 5 parts by mass with respect to 100 parts by mass ofthe PFSA resin component. Then, the resultant was passed through acolumn packed with a particulate cation-exchange resin particle tothereby nearly completely remove alkali ion components, to thereby makea mixed dispersion liquid (ASF3) in which ionic bonds of at least a partof the functional groups (sulfonic acid groups and alkaline nitrogenatoms) were generated.

The obtained mixed dispersion liquid was cast on a polyimide film as acarrier sheet by a well-known usual method, exposed to hot air at 120°C. (for 20 min) to nearly completely evaporate the solvent and dry tothereby obtain a membrane. The membrane was further subjected to a heattreatment in a hot air atmosphere under the condition of 160° C. for 10min to thereby obtain an electrolyte membrane of 25 μm in membranethickness. The variation rate of EWs before and after the heat treatmentof the obtained electrolyte membrane was about 0.2 to 0.3%. Theequilibrium moisture content of the obtained electrolyte membrane was12% by mass.

The maximum moisture content of the electrolyte membrane in water at 25°C. for 3 hours was 18% by mass.

As a result of carrying out a charge and discharge test by the similarmethod as in Example 1, the current efficiency (%)/the cell electricresistivity (Ω·cm²) was 98.2/0.95; and as a result of carrying out 200cycles of the charge and discharge as the durability test, the currentefficiency (%)/the cell electric resistivity (Ω·cm²) was 98/0.95, givingso small a variation, and exhibiting excellent oxidation resistance.

The amount of the fluorine ions eluted was measured by using theobtained electrolyte membrane, and was 0.01% of the whole amount offluorine in the immersed polymer.

Example 10

An electrolyte membrane was obtained by the similar method as in Example9, except for using Nafion DE2021 (made by Du Pont K. K., 20% solution,EW: 1,050) instead of the dispersion liquid (ASF2). The equilibriummoisture content of the membrane was 6% by mass, and the maximummoisture content was 14% by mass.

As a result of carrying out a charge and discharge test by the similarmethod as in Example 1, the current efficiency (%)/the cell electricresistivity (Ω·cm²) was 97.2/0.98. As a result of carrying out 200cycles of the charge and discharge as the durability test, the currentefficiency was 97% and the cell electric resistance was 0.99 Ω·cm².

The amount of the fluorine ions eluted was measured by using theobtained electrolyte membrane, and was 0.02% of the whole amount offluorine in the immersed polymer.

Example 11

An electrolyte membrane was obtained by the similar method as in Example9, except for using a PPE powder (made by Nihon Extron Co., Ltd.)instead of the PPS powder used in Example 9. The equilibrium moisturecontent of the membrane was 11% by mass, and the maximum moisturecontent was 18% by mass. As a result of carrying out a charge anddischarge test by the similar method as in Example 1, the currentefficiency (%)/the cell electric resistivity (Ω·cm²) was 98/0.95. As aresult of carrying out 200 cycles of the charge and discharge as thedurability test, the current efficiency was 97.8% and the cell electricresistance was 0.95 Ω·cm².

The amount of the fluorine ions eluted was measured by using theobtained electrolyte membrane, and was 0.01% of the whole amount offluorine in the immersed polymer.

Comparative Example 1

An electrolyte membrane was obtained by the similar method as in Example1, except for using Nafion DE2021 (made by Du Pont K. K., 20% solution,EW: 1,050) instead of the dispersion liquid (ASF1). The equilibriummoisture content of the membrane was 4% by mass.

As a result of carrying out a charge and discharge test by the similarmethod as in Example 1, the current efficiency (%)/the cell electricresistivity (Ω·cm²) was 94.5/1.20, in which the current efficiency wasin a lower level than in Examples 1 and 2. This is presumably becausethe electrolyte membrane of Comparative Example 1 had a low ionpermselectivity. As a result of carrying out 200 cycles of the chargeand discharge as the durability test, the current efficiency was 86.0%and the electric resistance was 1.30, also exhibiting inferiordurability.

The amount of the fluorine ions eluted was measured by using theobtained electrolyte membrane, and was 0.05% of the whole amount offluorine in the immersed polymer.

Comparative Example 2

An electrolyte membrane was obtained by the similar method as in Example1, except for using the dispersion liquid (ASF2) instead of thedispersion liquid (ASF1). The equilibrium moisture content of themembrane was 12% by mass, and the maximum moisture content was 23% bymass.

As a result of carrying out a charge and discharge test by the similarmethod as in Example 1, the current efficiency (%)/the cell electricresistivity (Ω·cm²) was 97.5/0.90. As a result of carrying out 200cycles of the charge and discharge as the durability test, the currentefficiency was 89.7% and the electric resistance was 1.18, exhibitinginferior durability.

The amount of the fluorine ions eluted was measured by using theobtained electrolyte membrane, and was 0.04% of the whole amount offluorine in the immersed polymer.

Comparative Example 3

As a result of carrying out a charge and discharge test by using Nafion112 (made by Du Pont K. K., membrane thickness: 50 μm) by the similarmethod as in Example 1, the current efficiency (%)/the cell electricresistivity (Ω·cm²) was 94.0/1.20. As a result of carrying out 200cycles of the charge and discharge as the durability test, the currentefficiency was 85.2% and the electric resistance was 1.30, exhibitinginferior durability.

The amount of the fluorine ions eluted was measured by using theobtained electrolyte membrane by the similar method as in Example 1, andwas 0.05% of the whole amount of fluorine in the immersed polymer.

In Table 1, the results of the above Examples 1 to 11 and ComparativeExamples 1 to 3 are shown.

TABLE 1 Polyelectrolyte Polyelectrolyte Polymer Membrane EquivalentAdditive Membrane Polymer Weight Terminal Content Composition ThicknessName MFI (g/10 min) (g/eq) Fluorination Kind (%) Name (μm) Example 1 A11.5 650 present — — ASF1 50 Example 2 Nafion — 1050 present — — — 50DE2021CS Example 3 A1 1.5 650 absent cerium 10 ASF2 50 nitrate Example 4Nafion — 1050 absent cerium 10 — 50 DE2021 carbonate Example 5 A1 1.5650 absent cobalt nitrate 14.7 ASF2 50 Example 6 A1 1.5 650 absentmanganese 13.8 ASF2 50 acetate Example 7 Nafion — 1050 absent cobaltnitrate 25 — 50 DE2021 Example 8 Nafion — 1050 absent manganese 25 — 50DE2021 acetate Example 9 A1 1.5 650 absent PPS 5 ASF2 25 Example 10Nafion — 1050 absent PPS 5 — 25 DE2021 Example 11 A1 1.5 650 absent PPE5 ASF2 25 Comparative Nafion — 1050 absent — — — 50 Example 1 DE2021Comparative A1 1.5 650 absent — — ASF2 50 Example 2 Comparative — — — —— — Nafion 50 Example 3 112 Polyelectrolyte Membrane Membrane Change andDischarge Test of Redox Flow Battery Membrane Maximum Current CellElectric Equilibrium Moisture Current Cell Electric EfficiencyResistivity Moisture Content Amount of Membrane Efficiency Resistivity(%) (Ω · cm²) Content (25° C.) Fluorine Ions Thickness (%) (Ω · cm²)(after 200 (after 200 (mass %) (mass %) Eluted (%) (μm) (initial)(initial) cycles) cycles) Example 1 12 23 0.001 50 97.5 0.90 97.3 0.90Example 2 6 14 0.002 50 94.5 1.20 94.0 1.20 Example 3 12 23 0.008 5097.5 0.90 97.3 0.90 Example 4 6 14 0.01 50 94.5 1.20 94.0 1.20 Example 512 23 0.01 50 97.5 0.90 97.3 0.90 Example 6 12 23 0.01 50 97.5 0.90 97.30.90 Example 7 6 14 0.02 50 94.5 1.20 94.0 1.20 Example 8 6 14 0.02 5094.5 1.20 94.0 1.20 Example 9 12 18 0.01 25 98.2 0.95 98.0 0.95 Example10 6 14 0.02 25 97.2 0.98 97.0 0.99 Example 11 11 18 0.01 25 98.0 0.9597.8 0.95 Comparative 4 — 0.05 50 94.5 1.20 86.0 1.30 Example 1Comparative 12 23 0.04 50 97.5 0.90 89.7 1.18 Example 2 Comparative — —0.05 50 94.0 1.20 85.2 1.30 Example 3

The present application is based on Japanese Patent Applications(Japanese Patent Application Nos. 2011-290097, 2011-290070, and2011-290077), filed on Dec. 28, 2011 in the Japan Patent Office, and aJapanese Patent Application (Japanese Patent Application No.2012-010245), filed on Jan. 20, 2012 in the Japan Patent Office, theentire content of which are hereby incorporated by reference.

INDUSTRIAL APPLICABILITY

The redox flow secondary battery according to the present invention islow in the electric resistance, high in the current efficiency, andexcellent in the durability as well.

The electrolyte membrane for a redox flow secondary battery according tothe present invention is excellent in the ion permselectivity, low inthe electric resistance, and excellent in the durability (mainly hydroxyradical oxidation resistance) as well, and is industrially applicable asa separation membrane for a redox flow secondary battery.

REFERENCE SIGNS LIST

-   1 POSITIVE ELECTRODE-   2 POSITIVE ELECTRODE CELL CHAMBER-   3 NEGATIVE ELECTRODE-   4 NEGATIVE ELECTRODE CELL CHAMBER-   5 ELECTROLYTE MEMBRANE-   6 ELECTROLYTIC BATH-   7 POSITIVE ELECTRODE ELECTROLYTE SOLUTION TANK-   8 NEGATIVE ELECTRODE ELECTROLYTE SOLUTION TANK-   9 AC/DC CONVERTER-   10 REDOX FLOW SECONDARY BATTERY

1-16. (canceled)
 17. An electrolyte membrane for a redox flow secondarybattery, comprising an ion-exchange resin composition comprising afluorine-based polyelectrolyte polymer having a structure represented bythe following formula (1):—[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1)wherein X¹, X², and X³ each independently represent one or more selectedfrom the group consisting of halogen atoms and perfluoroalkyl groupshaving 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZwherein Z represents a hydrogen atom, an alkali metal atom, an alkalineearth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄)wherein R₁, R₂, R₃, and R₄ each independently represent one or moreselected from the group consisting of alkyl groups and arene groups,when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² eachindependently represent one or more selected from the group consistingof halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groupshaving 1 to 10 carbon atoms; and a and g represent numbers satisfying0≦a<1, 0<g≦1, and a+g=1, b represents an integer of 0 to 8, c represents0 or 1, and d, e, and f each independently represent an integer of 0 to6 (with the proviso that d, e, and f are not 0 at the same time); andwherein in a test in which 0.1 g of the fluorine-based polyelectrolytepolymer is immersed in 50 g of a Fenton's reagent solution containing a3% hydrogen peroxide solution and 200 ppm of divalent iron ions at 40°C. for 16 hours, an amount of fluorine ions eluted detected in asolution is 0.03% or smaller of a whole amount of fluorine in a immersedpolymer.
 18. The electrolyte membrane for a redox flow secondary batteryaccording to claim 17, wherein in the test in which 0.1 g of thefluorine-based polyelectrolyte polymer is immersed in 50 g of theFenton's reagent solution containing the 3% hydrogen peroxide solutionand 200 ppm of divalent iron ions at 40° C. for 16 hours, the amount offluorine ions eluted detected in the solution is 0.002% or smaller ofthe whole amount of fluorine in the immersed polymer.
 19. Theelectrolyte membrane for a redox flow secondary battery according toclaim 17, wherein the fluorine-based polyelectrolyte polymer is aperfluorocarbonsulfonic acid resin (PFSA) having a structure representedby the following formula (2):—[CF₂—CF₂]_(a)—[CF₂—CF((—O—(CF₂)_(m)—SO₃H)]_(g)—  (2) wherein a and grepresent numbers satisfying 0≦a<1, 0<g≦1, and a+g=1; and m representsan integer of 1 to
 6. 20. The electrolyte membrane for a redox flowsecondary battery according to claim 17, wherein the fluorine-basedpolyelectrolyte polymer has an equivalent weight EW (dry mass in gramsper equivalent of ion-exchange groups) of 300 to 1,300 g/eq; and theelectrolyte membrane has an equilibrium moisture content of 5 to 80% bymass.
 21. The electrolyte membrane for a redox flow secondary batteryaccording to claim 17, wherein the electrolyte membrane is subjected toa heat treatment at 130 to 200° C. for 1 to 60 min.
 22. An electrolytemembrane for a redox flow secondary battery, comprising an ion-exchangeresin composition comprising a fluorine-based polyelectrolyte polymerhaving a structure represented by the following formula (1):—[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1)wherein X¹, X², and X³ each independently represent one or more selectedfrom the group consisting of halogen atoms and perfluoroalkyl groupshaving 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZwherein Z represents a hydrogen atom, an alkali metal atom, an alkalineearth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄)wherein R₁, R₂, R₃, and R₄ each independently represent one or moreselected from the group consisting of alkyl groups and arene groups,when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² eachindependently represent one or more selected from the group consistingof halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groupshaving 1 to 10 carbon atoms; and a and g represent numbers satisfying0≦a<1, 0<g≦1, and a+g=1, b represents an integer of 0 to 8, c represents0 or 1, and d, e, and f each independently represent an integer of 0 to6 (with the proviso that d, e, and f are not 0 at the same time); andwherein the ion-exchange resin composition comprises 0.1 to 20 parts bymass of a polyphenylene ether resin and/or a polyphenylene sulfide resinwith respect to 100 parts by mass of the fluorine-based polyelectrolytepolymer.
 23. The electrolyte membrane for a redox flow secondary batteryaccording to claim 22, wherein the fluorine-based polyelectrolytepolymer is a perfluorocarbonsulfonic acid resin (PFSA) having astructure represented by the following formula (2):—[CF₂—CF₂]_(a)—[CF₂—CF((—O—(CF₂)_(m)—SO₃H)]_(g)—  (2) wherein a and grepresent numbers satisfying 0≦a<1, 0<g≦1, and a+g=1; and m representsan integer of 1 to
 6. 24. The electrolyte membrane for a redox flowsecondary battery according to claim 22, wherein the fluorine-basedpolyelectrolyte polymer has an equivalent weight EW (dry mass in gramsper equivalent of ion-exchange groups) of 300 to 1,300 g/eq; and theelectrolyte membrane has an equilibrium moisture content of 5 to 80% bymass.
 25. The electrolyte membrane for a redox flow secondary batteryaccording to claim 22, wherein the electrolyte membrane is subjected toa heat treatment at 130 to 200° C. for 1 to 60 min.
 26. An electrolytemembrane for a redox flow secondary battery, comprising an ion-exchangeresin composition comprising a fluorine-based polyelectrolyte polymerhaving a structure represented by the following formula (1):—[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1)wherein X¹, X², and X³ each independently represent one or more selectedfrom the group consisting of halogen atoms and perfluoroalkyl groupshaving 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZwherein Z represents a hydrogen atom, an alkali metal atom, an alkalineearth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄)wherein R₁, R₂, R₃, and R₄ each independently represent one or moreselected from the group consisting of alkyl groups and arene groups,when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² eachindependently represent one or more selected from the group consistingof halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groupshaving 1 to 10 carbon atoms; and a and g represent numbers satisfying0≦a<1, 0<g≦1, and a+g=1, b represents an integer of 0 to 8, c represents0 or 1, and d, e, and f each independently represent an integer of 0 to6 (with the proviso that d, e, and f are not 0 at the same time); andwherein the ion-exchange resin composition comprises one or moreselected from the group consisting of a Ce-based additive, a Co-basedadditive, and a Mn-based additive.
 27. The electrolyte membrane for aredox flow secondary battery according to claim 26, wherein thefluorine-based polyelectrolyte polymer is a perfluorocarbonsulfonic acidresin (PFSA) having a structure represented by the following formula(2):—[CF₂—CF₂]_(a)—[CF₂—CF((—O—(CF₂)_(m)—SO₃H)]_(g)—  (2) wherein a and grepresent numbers satisfying 0≦a<1, 0<g≦1, and a+g=1; and m representsan integer of 1 to
 6. 28. The electrolyte membrane for a redox flowsecondary battery according to claim 26, wherein the fluorine-basedpolyelectrolyte polymer has an equivalent weight EW (dry mass in gramsper equivalent of ion-exchange groups) of 300 to 1,300 g/eq; and theelectrolyte membrane has an equilibrium moisture content of 5 to 80% bymass.
 29. The electrolyte membrane for a redox flow secondary batteryaccording to claim 26, wherein the electrolyte membrane is subjected toa heat treatment at 130 to 200° C. for 1 to 60 min.
 30. A redox flowsecondary battery comprising an electrolytic bath comprising: a positiveelectrode cell chamber comprising a positive electrode composed of acarbon electrode; a negative electrode cell chamber comprising anegative electrode composed of a carbon electrode; and an electrolytemembrane as a separation membrane to separate the positive electrodecell chamber and the negative electrode cell chamber, wherein thepositive electrode cell chamber comprises a positive electrodeelectrolyte solution comprising a positive electrode active substance;and the negative electrode cell chamber comprises a negative electrodeelectrolyte solution comprising a negative electrode active substance;wherein the redox flow secondary battery charges and discharges based onchanges in valences of the positive electrode active substance and thenegative electrode active substance in the electrolyte solutions;wherein the electrolyte membrane comprises an ion-exchange resincomposition comprising a fluorine-based polyelectrolyte polymer having astructure represented by the following formula (1):—[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1)wherein X¹, X², and X³ each independently represent one or more selectedfrom the group consisting of halogen atoms and perfluoroalkyl groupshaving 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZwherein Z represents a hydrogen atom, an alkali metal atom, an alkalineearth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄)wherein R₁, R₂, R₃, and R₄ each independently represent one or moreselected from the group consisting of alkyl groups and arene groups,when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² eachindependently represent one or more selected from the group consistingof halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groupshaving 1 to 10 carbon atoms; and a and g represent numbers satisfying0≦a<1, 0<g≦1, and a+g=1, b represents an integer of 0 to 8, c represents0 or 1, and d, e, and f each independently represent an integer of 0 to6 (with the proviso that d, e, and f are not 0 at the same time); andwherein in a test in which 0.1 g of the fluorine-based polyelectrolytepolymer is immersed in 50 g of a Fenton's reagent solution containing a3% hydrogen peroxide solution and 200 ppm of divalent iron ions at 40°C. for 16 hours, an amount of fluorine ions eluted detected in asolution is 0.03% or smaller of a whole amount of fluorine in a immersedpolymer.
 31. The redox flow secondary battery according to claim 30,wherein sulfuric acid electrolyte solutions comprising vanadium are usedas the positive electrode electrolyte solution and the negativeelectrode electrolyte solution.
 32. A redox flow secondary batterycomprising an electrolytic bath comprising: a positive electrode cellchamber comprising a positive electrode composed of a carbon electrode;a negative electrode cell chamber comprising a negative electrodecomposed of a carbon electrode; and an electrolyte membrane as aseparation membrane to separate the positive electrode cell chamber andthe negative electrode cell chamber, wherein the positive electrode cellchamber comprises a positive electrode electrolyte solution comprising apositive electrode active substance; and the negative electrode cellchamber comprises a negative electrode electrolyte solution comprising anegative electrode active substance; wherein the redox flow secondarybattery charges and discharges based on changes in valences of thepositive electrode active substance and the negative electrode activesubstance in the electrolyte solutions; wherein the electrolyte membraneis the electrolyte membrane for a redox flow secondary battery accordingto claim
 22. 33. The redox flow secondary battery according to claim 32,wherein sulfuric acid electrolyte solutions comprising vanadium are usedas the positive electrode electrolyte solution and the negativeelectrode electrolyte solution.
 34. A redox flow secondary batterycomprising an electrolytic bath comprising: a positive electrode cellchamber comprising a positive electrode composed of a carbon electrode;a negative electrode cell chamber comprising a negative electrodecomposed of a carbon electrode; and an electrolyte membrane as aseparation membrane to separate the positive electrode cell chamber andthe negative electrode cell chamber, wherein the positive electrode cellchamber comprises a positive electrode electrolyte solution comprising apositive electrode active substance; and the negative electrode cellchamber comprises a negative electrode electrolyte solution comprising anegative electrode active substance; wherein the redox flow secondarybattery charges and discharges based on changes in valences of thepositive electrode active substance and the negative electrode activesubstance in the electrolyte solutions; wherein the electrolyte membraneis the electrolyte membrane for a redox flow secondary battery accordingto claim
 26. 35. The redox flow secondary battery according to claim 34,wherein sulfuric acid electrolyte solutions comprising vanadium are usedas the positive electrode electrolyte solution and the negativeelectrode electrolyte solution.